Everyone with a gmail address or youtube account was strong armed into having a google+ account, which counts as their "userbase" regardless of whether or not any of those people actually made use of any google+ specific features.

There are over 3.3 billion G+ profiles, based on the March 2017 sitemaps files, though how many of those profiles are active is something of an open question.

About 9% of Google+ accounts ever posted any public data. That's roughly 300 million profiles there, though for many of them, the posting was at best minimal.

Based on publicly-visible posting activity, I and Stone Temple Consulting independently determined that there were ~4 - 16 million profiles publishing on a monthly basis or better in 1Q2015. Given other forms of participation, it's reasonable to assume a ~10x larger general active, but lurking, population, which gets us to about 40 - 160 million users.

Various other forms of population estimation come up with fairly similar statistics. Keep in mind that until 2007, an online community with >10m users was extraordinary of itself. A few were larger (Google's own Blogger had 200m MUA in 2008, larger than Facebook at the time: https://techcrunch.com/2008/12/31/top-social-media-sites-of-...).

The most interesting thing about the 52 million user statistic reported is that it is in the general ballpark of at least one set of estimates of G+ user counts. A number on which Google have been showing remarkably more candor as of late, though far from crystal clarity.

I personally had some dozens thousands followers from all over the Earth on Google+, communicated to them actively through commenting and have occasionally met some of them offline when they traveled to my country.