[bdlbdl]

It's because a Facebook connected user is just a much more valuable user to your service. You have access to their friend list, photos, likes, etc. And it's easy.

Sure, they could do the work to let non-FB connected users in as well, but given the scale of FB and the value of FB data, it's really hard to justify that work until you've hit product market fit.

When you only have one path to sign up and register it's really easy to focus and make that path as high quality as possible. Later you can add support for the people that want to use your product but don't have FB.