Because, it's a lot easier to just sign up using your Facebook account rather than trying to remember yet another password. But I don't want every little site to know my personal information. It's none of their business. I just want to log in quickly.

Logging in to a site provides more value than just sharing personal information. For me, I rarely, if ever, want a site to know my personal information. That's why I rarely use Facebook login service. Now I will.