14 DAY TRIAL // Numbers are all over the place and it's hard to get a clear picture of how well Google+ is doing, or not. Even Google's official numbers aren't entirely enlightening.
Still, it does seem like December was a break-out month for the site. Hitwise is seeing a 55 percent increase in visits in the US in December up from the previous month.
In total, Google+ saw 49 million visits in the US alone last month. Note that Hitwise is counting total visits, not visitors, so that number should be considerably smaller.
What's more, it may not necessarily mean that a lot more people started visiting the site, but it does mean that people are coming to the site more and more often.
Corroborated with other analysis though, it does seem like December was a big month for growth at the site, both in terms of user numbers and visits. Paul Allen, who has been keeping an eye on Google+ user number growth, estimated that the site would end the year with 65 million registered users.
Again, this number is not entirely representative since a lot of people may have signed up and then stopped visiting. A significant number is bound to have done just that, it's a typical trend for any new, suddenly popular site.
There doesn't seem to be any particular reason for Google+'s sudden rise in usage and popularity in the last month, but there's no indication that this is a permanent trend or not.
Google+ will undoubtedly continue to gain new users, Google is too big and it is putting too much of an effort for it not to happen, but the real question is whether people come back and become regular users. So far, there is little data on this, arguably the most important metric.