Ease of signup. If all I have to do is click a "sign in with my facebook account" and I don't have to sign up for yet another site, I might be compelled to join

Ease of signup. If all I have to do is click a "sign in with my facebook account" and I don't have to sign up for yet another site, I might be compelled to join your site and thus be your customer. Not saying facebook login is the best way to do this, but as a user, it's comfortable enough.