I would bet that is a relatively small portion of their addressable market. Sure, they will eventually want to support those users, but for an initial launch, it is not worth spending time building an authentication system when they can just use Facebook to reach the majority of their users.
Huh? They have to build an authentication system for Facebook. What's the difference? It's just a matter of adding a username, and password rather then facebook stuff?
Facebook provides a lot of public info (profile pic, first name and last name) that would otherwise be really annoying for the user to type in. Of course, to each his/her own.
