I've considered GitHub and Bitbucket logins to be great options if you have a product that targets developers, but want to try and avoid (for a while, or forever) handling usernames/passwords/password resets/etc yourself. Both seem to have a general sense of "trust" that many do not have for Facebook.

I'm implementing Google & GitHub (w/ BitBucket to follow just after) OAuth2 flows on a small project I'm building now. Both services are extremely pervasive amongst the specific developers I'm targeting, so I consider users unwilling to sign-in with either of those services a minority (but one that I won't ignore, if they clamour for it!). I just had to draw the line in the sand somewhere if I'm going to get this out the door this summer (winter, here).