Firstly, Facebook really do "move fast and break stuff," and dealing with their APIs is both slow and aggravating. The app I'm working on has a lot of social fu

Firstly, Facebook really do "move fast and break stuff," and dealing with their APIs is both slow and aggravating. The app I'm working on has a lot of social functionality, which is why I'm bothering. (You may have similar plans). I've never implemented Google account login, but if I didn't care about the social stuff I'd be leaning towards that. Are you doing a "hard launch" with lots of PR, etc? If so, I would be thinking very hard about the risks in being FB-only. A lot of people will never visit your site again. If not, then you have more freedom to experiment. I'd be inclined to go with my gut, but: metrics, metrics, metrics. From day one you should know how many people are being pushed to authorize your application to know about their Facebook identity, and how many of them are coming back after clicking the "Authorize" button. Facebook has a dashboard which I haven't investigated, but it may be good enough for you to avoid implementing your own metrics.