Sites come and go. Facebook may be around for years to come, but then again... Data is only as good as their server infrastructure. Most cheap or free web services have built their data infrastructure on inexpensive disk arrays, which have serious vulnerabilities. Their RAID array may be protected, but it is very possible for the entire array to get corrupted during upgrades, etc. and data loss will occur.
Another vulnerability is security, another is physical damage. Three months ago, the Authorize.net data center in Seattle had a small electrical fire but the data center was shutdown for 24 hours and customers could not access their files at all during that time. Google was down a couple of weeks ago for two or three hours one night - yes it's 2AM but I want my data!!
The smart thing to do is not worry about these things, by having all of your web files automatically backed up to a second or third location an inexpensive service like FilesAnywhere - these sites let you USE your online backed up files for other purposes also. So you get the peace of mind from having your files backed up - from Facebook, MySpace, LinkedIn, Twitter, GeoCities, Google, or any other web service - plus you can use those files anytime from a network drive mapping, or by sharing links to the entire folder of files.
Of course, most people are careful and have a copy of their files at least on a local USB drive right? Hello! What happens when that certain someone at your house "accidentally" deletes your entire root folder on your USB drive, and by chance, your favorite web service closed last week where you had second copies of your files? This is when you will be glad to have automated backups setup to copy everything offsite to FilesAnywhere or a secure FTP site.
The idea is to have several secure copies, in different locations, to fully protect your digital life. If you already have an offsite backup provider, it's still the safest approach to have more than one. Even the offsite backup companies have lifespans. Look at Xdrive, Yahoo Briefcase, HP's online backup service - all have closed in the last year and without much warning.