actually I'm pretty sure it's because it's effectively in beta and the servers wouldn't be able to handle the several billion people that would sign up in the first month or so if they made it a free-for-all
in fact the video chat service almost crippled them and they had to stop letting people send invites for a while
as for what it is, I can't really say for sure. It's probably going to advertise itself as the social networking site that doesn't make you feel like your privacy is constantly at dire peril