True but those people aren't a good fit for their service to begin with. Non-facebook users are not, by default, bad fits. Big difference. Hell I would bet at least some of FBs user base is also in that category of crap internet.

Cutting off arbitrary users who aren't bad fits for you is a good way to limit your user base. Sure it's 700-800 million people, but not all of the ones who use FB are going to want your service, and if it is any good some who don't use FB will. Why limit yourself that way? Especially when they already supported non-FB accounts for everything but the social features, so the work had already been put in.