Very obvious answer for this - because Google+ was seeded by Google employees and their social networks, and these skew overwhelmingly male. This is the Silicon Valley tech industry, afterwards.
I suspect that the gender balance will become much more equal as those initial contacts branch out and can send their own invites, simply because of regression to the mean.
I suspect that the gender balance will become much more equal as those initial contacts branch out and can send their own invites, simply because of regression to the mean.