The reason people pick Facebook only is because they think that losing users that don't use Facebook is better than having to implement your own account system.
Authorization and account management is really easy to screw up. If you leave it to Facebook, you'll save a lot of time. The only question is whether or not it's worth it.
Or, if I'm not, I've polluted your user database with one of my throw-away Facebook accounts just so I could see what was behind that search result that caught my eye.
userid, password. What part of that do you not understand?
Surely there's a way to avoid implementing your own account system that doesn't drag in the massive unrelated architecture of a Facebook, Google, or Twitter. You don't even need SSO like OpenID.
Is there a company that just sells a no-frills user enrollment and login service and also provides strict isolation between sites? (I.e., they resist the temptation to leverage their aggregate user base.)
Authorization and account management is really easy to screw up. If you leave it to Facebook, you'll save a lot of time. The only question is whether or not it's worth it.