Gmail can be successful regardless of whether or not your friends use it.

A social network is only as successful as the number of friends using it.

Facebook did well on this by opening up things campus by campus ensuring that all your friends in your university were all allowed to use it at the same time.

Gmail used to be infinitely better than everything else on the market. So closing it it off made it only more scarce and sought after than it already was. Now if plus is even finitely better than everything else, so that artificial scarcity will make it seem more valuable, is debatable. It's a rather risky bet Google has already lost once with Wave.

I can remember people selling GMail invites for a decent amount at the very beginning of their launch.

Email isn't exactly this type of walled-garden social however, so it's not an apt comparison.

That's why I said social is not like email. I'm more comparing the Google+ rollout to Wave, since some of the features seem to be similar to Wave.

I was referring to "This thing needs to be free and massively available like Gmail."

Yeah massively available like Gmail - I meant after it opened up from invite-only status.

Email is easy to bootstrap because any client can talk using SMTP. Social does not have a protocol - although that might be cool if it did so we didn't have the "all your data are belong to us" problems of Facebook and Google.