It turns out Google+ is a virtual ghost town compared with the site of rival Facebook Inc., which is preparing for a massive initial public offering. New data from research firm comScore Inc. shows that Google+ users are signing up—but then not doing much there.

Visitors using personal computers spent an average of about three minutes a month on Google+ between last September and January, versus six to seven hours on Facebook each month over the same period, according to comScore, which didn't have data on mobile usage.

Behind the lack of engagement are Google's difficulties in differentiating Google+ from Facebook.