Do you know all those cute tests on Facebook? Which Star Trek character are you? Who’s going to help you rob a bank? Did you know they’re spamming you and your friends?
Remember this simple principle: TANSTAAFL. There ain’t no such thing as a free lunch. These sites are giving you a little diversion in exchange for getting access to your email address and your friends list.
Thus, you get spam. And if your friends have made their email addreses accessible to onlookers, they get spam too. Plus whatever else can be done with your personal info. These guys are out to make money, and you have just given them a boatload of information, which they will now use to their own end.
Better think twice about letting some web app tell you which Beatle you are. Your mother should know. So should you.