[AgentME]

Only if the facebook account gets compromised. If some random site you use facebook to log into gets compromised, your facebook account and every other site you log into with it is still safe.

This is not exactly worse than web sites that allow you to sign up with an email address and request password resets: if your email address is compromised, then so are all your accounts. Actually, this situation is a bit worse, because there's also the chance that the user will choose the same password as they use with their email address or other accounts, so if the site they're logging into gets compromised, then their other accounts and email may be able to be compromised too.