So according to a Google+ Google engineer, Ivan Yudhi, it is around the fifth year birthday for the launch of Google+.
He wrote on Google+:
So my G+ anniversary isn't actually until July 11th (the day I made my first post), but I checked Wikipedia and it was stated that Google+ was launched in June, so might as well do it now.
Google launched it today, June 28, 2011, here is their blog post from then.
It is sad to see it not live up to what the company expected for it.
But I personally still enjoy Google+, although it has way less people on it now than it did two years ago.
Happy birthday Google+!
Forum discussion at Google+.