When you sign up for Facebook, you enter into a bargain. You share personal information with the site, and Facebook agrees to obey your wishes when it comes to who can see what you post. At the same time, you agree that Facebook can use that data to decide what ads to show you, The New York Times reports. It is a complicated deal that many people enter into without perhaps fully understanding what will happen to their information. It also involves some trust — which is why any hint that Facebook may not be holding up its end of the bargain is sure to kick up plenty of controversy. The latest challenge to that trust came on Monday, when Facebook acknowledged that some applications on its site, including the popular game FarmVille, had improperly shared identifying information about users, and in some cases their friends, with advertisers and Web tracking companies. The company said it was talking to application developers about how they handled personal information, and was looking at ways to prevent this from happening again.