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To hear Google chief executive Larry Page tell it, Google+ has become a robust competitor in the social networking space, with 90 million users registering since its June launch.

But those numbers mask what is really going on at Google+. It turns out the social network is a virtual ghost town compared with the site of rival Facebook, which is preparing for a massive initial public offering. New data from research firm comScore reveal that Google+ users are signing up -- but 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 did not have data on mobile usage.

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

When Google+ launched last year, the internet search giant positioned it as a Facebook competitor where people can share comments, articles, photos and videos with specific groups of friends and contacts.

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While Google+ has some original features -- including "Hangouts," which lets people start a video conference with up to 10 people -- analysts and some consumers say the distinction is not enough to lure Facebook members away and persuade them to build a network of contacts from scratch on Google+.

"Nobody wants another social network right now," Brian Solis, an analyst at social-media advisory firm Altimeter Group said. For those who already use Facebook, "Google hasn't communicated what the value of Google+ is," he said.

Google executives downplay the direct comparison to Facebook, which has 845 million monthly active users. They have repeatedly said they are making a long-term bet on the initiative, and the company has yet to build up some of the weapons that made Facebook successful, including encouraging app development.

Read more on social networking and Google+ at The Wall Street Journal.