However, as lawyer Evan Brown correctly explains in Brodkin’s article, failing to take down works upon receipt of a DMCA notice does not automatically make one guilty of infringement. It just means they can’t directly avail themselves of the safe harbors in the DMCA. They can still make plenty of other arguments for why they have not broken the law. And Brown suggests that there are many reasons why they’re likely to prevail:

I don’t agree with Evan Brown’s assessment of material contribution. If I understand the facts correctly, a Twitter user uploaded a copyrighted image to Twitter’s servers, and then the user composed a tweet that included the image. So it’s more than just a link to an infringing image that’s hosted elsewhere since Twitter actually hosts the image as well as the tweet.

The DMCA safe harbors are not in force since Twitter ignored a takedown notice. I think it’s safe to say that Twitter has knowledge of the infringement since it received the takedown notice (which imparts knowledge, actual or constructive). So it boils down to whether Twitter is materially contributing to the infringement. I’d look to the Netcom-Napster-Amazon.com line of cases:

Where a defendant has knowledge of the primary infringer’s infringing activities, it will be liable if it ?induces, causes or materially contributes to the infringing conduct of? the primary infringer. Gershwin Publishing, 443 F.2d at 1162. Such participation must be substantial. Apple Computer, Inc. v. Microsoft Corp., 821 F.Supp. 616, 625 (N.D.Cal.1993), aff’d, 35 F.3d 1435 (9th Cir.1994); Demetriades v. Kaufmann, 690 F.Supp. 289, 294 (S.D.N.Y.1988).

Providing a service that allows for the automatic distribution of all Usenet postings, infringing and noninfringing, goes well beyond renting a premises to an infringer. See Fonovisa, Inc. v. Cherry Auction, Inc., 847 F.Supp. 1492, 1496 (E.D.Cal.1994) (finding that renting space at swap meet to known bootleggers not ?substantial participation? in the infringers’ activities). It is more akin to the radio stations that were found liable for rebroadcasting an infringing broadcast. See, e.g., Select Theatres Corp. v. Ronzoni Macaroni Corp., 59 U.S.P.Q. 288, 291 (S.D.N.Y.1943). Netcom allows Erlich’s infringing messages to remain on its system and be further distributed to other Usenet servers worldwide. It does not completely relinquish control over how its system is used, unlike a landlord. Thus, it is fair, assuming Netcom is able to take simple measures to prevent further damage to plaintiffs’ copyrighted works, to hold Netcom liable for contributory infringement where Netcom has knowledge of Erlich’s infringing postings yet continues to aid in the accomplishment of Erlich’s purpose of publicly distributing the postings. Accordingly, plaintiffs do raise a genuine issue of material fact as to their theory of contributory infringement as to the postings made after Netcom was on notice of plaintiffs’ infringement claim.

RTC v. Netcom, 907 F.Supp. 1361, 1375 (N.D. Cal. 1995) (emphasis added).

Under the facts as found by the district court, Napster materially contributes to the infringing activity. Relying on Fonovisa, the district court concluded that ?[w]ithout the support services defendant provides, Napster users could not find and download the music they want with the ease of which defendant boasts.? Napster, 114 F.Supp.2d at 919?20 (?Napster is an integrated service designed to enable users to locate and download MP3 music files.?). We agree that Napster provides ?the site and facilities? for direct infringement. See Fonovisa, 76 F.3d at 264; cf. Netcom, 907 F.Supp. at 1372 (?Netcom will be liable for contributory infringement since its failure to cancel [a user’s] infringing message and thereby stop an infringing copy from being distributed worldwide constitutes substantial participation.?). The district court correctly applied the reasoning in Fonovisa, and properly found that Napster materially contributes to direct infringement.

A&M v. Napster, 239 F.3d 1004, 1022 (9th Cir. 2001) (emphasis added).

Our tests for contributory liability are consistent with the rule set forth in Grokster. We have adopted the general rule set forth in Gershwin Publishing Corp. v. Columbia Artists Management, Inc., namely: ?one who, with knowledge of the infringing activity, induces, causes or materially contributes to the infringing conduct of another, may be held liable as a ?contributory? infringer,? 443 F.2d 1159, 1162 (2d Cir.1971). See Ellison, 357 F.3d at 1076; Napster, 239 F.3d at 1019; Fonovisa, Inc. v. Cherry Auction, Inc., 76 F.3d 259, 264 (9th Cir.1996).

We have further refined this test in the context of cyberspace to determine when contributory liability can be imposed on a provider of Internet access or services. See Napster, 239 F.3d at 1019?20. In Napster, we considered claims that the operator of an electronic file sharing system was contributorily liable for assisting individual users to swap copyrighted music files stored on their home computers with other users of the system. Napster, 239 F.3d at 1011?13, 1019?22. We stated that ?if a computer system operator learns of specific infringing material available on his system and fails to purge such material from the system, the operator knows of and contributes to direct infringement.? Id. at 1021. Because Napster knew of the availability of infringing music files, assisted users in accessing such files, and failed to block access to such files, we concluded that Napster materially contributed to infringement. Id. at 1022.

The Napster test for contributory liability was modeled on the influential district court decision in Religious Technology Center v. Netcom On?Line Communication Services, Inc. (Netcom), 907 F.Supp. 1361, 1365?66 (N.D.Cal.1995). See Napster, 239 F.3d at 1021. In Netcom, a disgruntled former Scientology minister posted allegedly infringing copies of Scientological works on an electronic bulletin board service. Netcom, 907 F.Supp. at 1365?66. The messages were stored on the bulletin board operator’s computer, then automatically copied onto Netcom’s computer, and from there copied onto other computers comprising ?a worldwide community? of electronic bulletin board systems. Id. at 1366?67 & n. 4 (internal quotation omitted). Netcom held that if plaintiffs ?*1172 could prove that Netcom knew or should have known that the minister infringed plaintiffs’ copyrights, ?Netcom[would] be liable for contributory infringement since its failure to simply cancel [the former minister’s] infringing message and thereby stop an infringing copy from being distributed worldwide constitute[d] substantial participation in [the former minister’s] public distribution of the message.? Id. at 1374.

Although neither Napster nor Netcom expressly required a finding of intent, those cases are consistent with Grokster because both decisions ruled that a service provider’s knowing failure to prevent infringing actions could be the basis for imposing contributory liability. Under such circumstances, intent may be imputed. In addition, Napster and Netcom are consistent with the longstanding requirement that an actor’s contribution to infringement must be material to warrant the imposition of contributory liability. Gershwin, 443 F.2d at 1162. Both Napster and Netcom acknowledge that services or products that facilitate access to websites throughout the world can significantly magnify the effects of otherwise immaterial infringing activities. See Napster, 239 F.3d at 1022; Netcom, 907 F.Supp. at 1375. The Supreme Court has acknowledged that ?[t]he argument for imposing indirect liability? is particularly ?powerful? when individuals using the defendant’s software could make a huge number of infringing downloads every day. Grokster, 545 U.S. at 929, 125 S.Ct. 2764. Moreover, copyright holders cannot protect their rights in a meaningful way unless they can hold providers of such services or products accountable for their actions pursuant to a test such as that enunciated in Napster. See id. at 929?30, 125 S.Ct. 2764 (?When a widely shared service or product is used to commit infringement, it may be impossible to enforce rights in the protected work effectively against all direct infringers, the only practical alternative being to go against the distributor of the copying device for secondary liability on a theory of contributory or vicarious infringement.?). Accordingly, we hold that a computer system operator can be held contributorily liable if it ?has actual knowledge that specific infringing material is available using its system,? Napster, 239 F.3d at 1022, and can ?take simple measures to prevent further damage? to copyrighted works, Netcom, 907 F.Supp. at 1375, yet continues to provide access to infringing works.

Perfect 10 v. Amazon.com, 508 F.3d 1146, 1171-72 (9th Cir. 2007) (emphasis added).

Since Twitter has knowledge that specific infringing images are available using its system and can take simple measures to prevent infringement yet continues to provide access to the images, it can be held liable as a contributory infringer.