Sites that only allow signup with Facebook kind of remind me of restaurants from the 30s with ‘Whites only’ signs outside.
I’m not suggesting that only allowing Facebook members to use your site is racist, or as terrible a crime as ‘whites only’ policies; but it is as stupid, shortsighted and pointless.
Just like there was no real reason (besides bigotry) for business owners to refuse service to people who weren’t the ‘right’ colour, only allowing Facebook members to use your service is an arbitrary restriction. Why create false barriers to entry?
Let’s not.