You are indeed delegating the identification and authentication to one point, and - by definition - introducing a single point of failure, yes. Then, it depends

You are indeed delegating the identification and authentication to one point, and - by definition - introducing a single point of failure, yes. Then, it depends how well your "single point" is protected. I definitely don't recommend making your social network account into a single-login point; if anything, using a dedicated, well-secured login service (such as a few of the OpenID providers) would be a better fit. In social networks, an account compromise is more a question of "when" than a question of "if."