That's not granular enough an explanation. They were strategic about who they let join and when, and that strategy involved admitting entire social groups at once.

Absolutely agreed. My point is just that it's not an entirely unintuitive idea in the general case.

Facebook used to grow methodically. There is no sensible answer of not letting people join a social community. Creating a new profile and looking for friends should be the only cost of users. It sucks to force people looking for invitations because Plus got no community. Plus got no killer feature like Gmail. Plus is not so cool, also.

I see that current Facebook users have the motivation to migrate. It's a good thing. But centralized, dominant social websites don't work anymore.