I think the invites are better in this case.
The author made a similar point of Google+.
Essentially, by not letting anyone in right now, and only having invites. That means no one can come in now and see it as a barren wasteland.
When they send out the invites, they'll probably send it to all the people who registered at the same time (hopefully a lot by that time) and when they all join on at once, they all see each other online. No wasteland problem.
2 and 3 and really valid though.