I feel like the startup community has skipped the conversation about using facebook or twitter for login and gone straight to this. There are a lot of apps now that aren't particularly social outside of interactions within the app[0] and require me to use facebook or twitter to log in. I don't want either of those to be my identity on other sites.
[0] And those interactions don't depend on an outside social graph at all. I'm thinking of Anyasq in particular here.
I'm not really convinced its in startups' best interests, either.
Being so limited, that is. Letting users get started without first creating yet another account is certainly awesome, and getting rid of that friction is undoubtedly a huge component of the success of sites like Stack Overflow.
But going with only one or two of the social behemoths seems unwise. It means you've decided to hitch your applecart to a business that, by virtue of being in the social space, could very easily become a competitor. Or decide to bless one of your competitors in some way that disadvantages you. Now, that's one thing if it's another startup with which you're forming a strategic partnership. But when it's one of the Facebooks or Twitters of Googles of the world, you're dreaming if you think you aren't doing business at the King's pleasure.
And with so many OpenID providers out there, it's not like it really takes any effort to give yourself some flexibility. So it's hard to fathom why any thoughtful, ambitious entrepreneur wouldn't want to do that.
Using Facebook/Twitter/Any-Other-Existing-Login is a good way to speed up the experience for new users, however, this should then create a new profile and always give the users a choice on how they login/maintain their profile with you.
I fully understand providing the option. "Create an account, or just log in with Facebook" is good for the user. My objection is when the "create an account" option isn't there and I can only use the site by logging in through a third party.
Anyasq is an example of this; you can only log in with facebook or twitter.
[0] And those interactions don't depend on an outside social graph at all. I'm thinking of Anyasq in particular here.