The Null Device
Facebook is in the news again, with (so far) the first known instance of a Facebook application being used to install adware on users' PCs. If your friends invite you to install the "Secret Crush" application, you accept, and you are using Windows, then the application will install the Zango adware program on your PC, not to mention arm-twist you into spamming your friends with requests to add it.
If Secret Crush actually needs you to click buttons to invite your friends to add it, the criminal scumbags who designed it have missed a trick; some other applications, such as RockYou's Super Wall and related applications, are able to send messages to randomly selected individuals from a user's friend list, purporting to be that user and asking to be installed to see a message from them, without the user's intervention. (I once found in my notifications the notice that I had messaged three randomly-chosen people, whose relationships to me have nothing in common, inviting them to install Super Wall. Soon after that, Super Wall was no longer installed on my page.)
The issue of data portability, or who owns your personal information and friend lists online, has entered the news recently as Facebook deleted the account of blogger Robert Scoble for using a script to automatically fetch his contact list, in violation the site's terms of service (which prohibit scripts, as they can be used for spamming and such). Scoble's account has been reinstated, on the proviso that he doesn't do it again, but not before raising an outcry on his high-profile blog.