When I first got on Facebook I fell prey to several of the applications that fling themselves at you like cheap whores at a pimp convention. Football, poker, calendars, weRead, Hammerfall, and everybody's favorite, Mafia Wars. Each of them asked for permission to access my profile and despite a twinge of unease, I clicked Yes.
After a time I started to understand what was going on; these applications weren't built by Facebook. Rather, they were being developed by companies with ties to advertising. In some of the game apps you could get points (or money or whatever currency the game used) for agreeing to this or that invasion of your privacy. Invite your Facebook friends to play! Give us your address! A now-famous one was the IQ test that required you to give up a cellphone number to get the results. Buried in the fine print (which could only be seen once you scrolled down the page) was a notice that the user agreed to a monthly subscription to a texting service that cost up to $19.99 a month.
I stopped using all of these application with the exception of poker, which I use to play with friends and family online since we can't get together in person. The bottom line is, if you're ever asked to give up any personal information like phone numbers, addresses, credit card numbers, etc., don't. If you do, keep a close eye on your phone bills and credit cards.
For an interesting inside look at how these scams get perpetrated and the future of advertising on social networking sites like Facebook, go here.