It's not like it's going to remain this way permanently, a lot is still under development. It's not just about infra scale, they are still hammering out things like how moderation will work, and that is the sort of thing best to iron out before the floodgates are opened.
Doesn't matter how good your social network product is if you don't catch the network effect early on. They are loosing a brutal battle against a billion dollar corp.
