When I see startups that only offer FB Connect as the singular sign in option, I always get squeamish.
I tell founders they should support FB Connect but they also support Twitter sign-in as well as giving users an option to sign in the good old fashioned way : user id & password.
For me it’s as simple as this: when you require a Facebook sign-in, you are essentially compelling me to buy into Facebook and the considerable baggage that goes along with Facebook to get any sort benefit from your service. For the (I suspect rapidly growing) portion of us who only grudgingly maintain any sort of Facebook presence, it’s like asking us to reaffirm our commitment to a religion before being allowed to eat in your restaurant. As Bijan suggests, it’s often enough of a battle to get people to buy into your new thing—don’t require people to buy into Facebook as well.