Despite the fact that it's been hard to get Google+ likely has more than 25 million users. The number is an estimate since Google hasn't released any data, though you can read the post I just linked to in order to see how it was estimated.
Seems like 25 million users is a reasonable guestimate at the moment and the count could be significantly greater.
Also for those of you wanting in it looks as though you might not need an invitation. Google+ seems to be open to registration at the moment.
The numbers seem pretty impressive so far. Granted it's a far cry from the half a billion or so Facebook users, but Google+ has only been around a few weeks and has been met with mostly positive reviews.
I wouldn't suggest that Facebook is going away any time soon, but Google seems to have managed to build a social network that isn't going anywhere. It's likely still filled with the tech savvy early adopters, but every social network typically starts there.
Definitely continues to be something worth paying attention to.
Added: Looks like the estimate, as reasonable as it sounded, wasn't quite accurate. I just found an article talking about Google's Q2 financial results in which Larry Page claims the numbers are much lower