eMarketer points to a search marketing study from SEOmoz estimating that only 54.9% of online marketers worldwide said Google+ was one of their top five most-used sites for social media marketing, compared with 87.7% who cited Facebook and 82.7% who cited Twitter. Who is using Google+, considering that Google estimates 100 million active monthly users out of about 400 million people worldwide who have signed up for the Google+ service? While eMarketer doesn't provide all the answers, the data yields some insights.