Using FB as a login is really a usability thing more than a time saving issue. The friction of having to choose yet another password to sign up for an account

Using FB as a login is really a usability thing more than a time saving issue. The friction of having to choose yet another password to sign up for an account is negated somewhat by using a FB/Twitter/Google-type oAuth/OpenID solution. I'm a big fan of federated login and prefer to see it on sites where possible. But equally, I'm not sure I agree about making your site completely dependant on FB. Although if the app also requires social graph, then they're not really losing out all that much are they?