My wife got an email from someone in her blog circle. That email included a random link at the bottom inviting her to Google+. It was not a Google+ email invitation, it was just a "hey, fill out this questionnaire" email. The blog system they use is Blogger (owned by Google), so that may have had something to do with it too. But after reading about the low percentage of female users, I wonder if Google didn't manage to find some extra memberships laying around that it happened to hand out to female users of their other services.