Just a point of clarification - recordable and rewritable CDs in Canada are still subject to this levy. There is no "Data" vs. "Audio" CD difference that was suggested above.
The law that created the CPCC says that the levy shall apply to all "media" that are "normally" used for the purposes of recording audio.
"Normally" in this context does not mean "typically" or "usually", just that the use would be considered "normal". This is why DVDs aren't subject to this levy - although they could record audio just as easily, this isn't a "normal" use. There aren't many audio-only DVD devices on the market.
Under this rule, media cards and hard drives would absolutely qualify for the levy, and almost did. It took a court ruling to determine that these were in fact "devices", not "media", and therefore didn't qualify for the levy.
Finally, why only audio? Why not video? Because the entire idea was dreamed up by executives and parliamentarians who wanted to compensate the music labels for all those mix tapes and home-burned CDs. Napster wouldn't hit it big for another 4 or 5 years.
Patents are only laid open to the public 18 months after they have been filed, and even at this point many companies haven't filed the assignment papers yet. Therefore, simply searching for 'Twitter' or another corporate name will not yield any results. Searching by any known inventors is much more effective.
Also, there's a quirk in US law that says if you disclaim the right to file patents internationally for your US patent, you don't have to publish them at all until they issue. This, of course, takes years.
Perhaps Twitter is trying something different, but the simpler explanation is probably that we just haven't seen their patents yet.