I must say, Google's decision to make Google+ invite only was puzzling at first but its quite obvious now isn't it?

It's much like how Facebook grew by restricting its service only to the top schools. This gained more hype for them as people saw that the 'elite' were using facebook and loving it.

Now we have personal invites to those who have a strong voice in the tech industry generating lots of good publicity for them. I'm not saying your a Silicon Valley elite if you have google+ account. But I think that the decision to keep google+ a private was more marketing, then a technical decision.