I can't say I blame them for not wanting to suddenly have to firefight 1 million users on an immature platform.

Aware that your issue is the approach and the fact that invites are the main way of scaling this rather than just issuing random waitlists, but there is an issue about networks which I assume they are needing - i.e. users already on there will want their friends on there with them, as it is a social network.

The issue about creating networks of people that know each other and interact is probably pretty key to building a real social network - see how Facebook rolled out by college campus.