"With around 60% of users identifying themselves as web developers or software engineers [...]"
That's probably the more interesting point
That's probably the more interesting point
note: these are my speculations.
According to the BLS there are 3.2 million Americans in "Computer and mathematical occupations" (http://www.bls.gov/news.release/ocwage.htm), and there seems to be a reasonable consensus that 12 million is an upper bound for the world (http://stackoverflow.com/questions/453880/how-many-developer...). There's no way 6 million are on Google+ already.
However that wasn't my point, neither of the sources they cite (findpeopleonplus.com, socialstatistics.com) back up this assertion. If you look at findpeopleonplus, it says out of the 947,996 people it has that 65,733 are an engineer/developer type. That's less than 10%.
The only thing that makes any sense is that it's 60% of the people who listed their occupation. Even that seems high, but it might be plausible given that the initial invitees would have been the friends of Googlers.
The other 40% are social media consultants.