Google+ is either doing great or is a complete disaster, depending on who you believe. If you ask Google, hardly an unbiased party, it's a complete success with some 100 million users at some point.
If you look at some stats revealed by comScore though, things are significantly less rosy. One important metric, ignored by Google so far, is engagement. Time spent on the site is an easy way of gauging that.
Google+ users spend an average of three minutes per month on the site, at least they did in January, comScore says. Meanwhile, they spend eight minutes on MySpace, 17 on LinkedIn and a whooping 405 minutes on Facebook. Pinterest and Tumblr manage to keep people occupied for about 98 minutes per month.
The figures are not accurate to the minute and it may very well be that a core audience of Google+ spends a lot more on the site. But the vast majority of users sign up and forget about it.