Comment by falcor84

I just found this October 2011 post reporting about this change as it happened (4 months after G+'s launch in June 2011):

https://waxy.org/2011/10/google_kills_its_other_plus/

According to Wikipedia, G+ usage kept growing from about 40M that October to 90M by the end of 2011 and then to hundreds of millions over the next few years, but the reporting methodology seems very inconsistent.

https://en.wikipedia.org/wiki/Google%2B