[broot]

I think you're right, the invite window doesn't matter, but like the first 100 days of a presidency, it's a solid enough chunk of time to think about what Google+ has accomplished, and I would argue that the answer is "not much."

The invite window won't kill G+, but apathy will. About a fifth of my Facebook friends are on it, but only 3 out of a hundred post anything. A successful competitor would need to stand for something other than "well, it's not Facebook."

It may find a niche as a long-form twitter (which is something I can't believe Facebook waited so long to do) but the initial excitement has run its course.