Google+ is catching up on a lot of fronts to Facebook, but it's still lagging in one key metric: Time spent.

The average user spent 6 minutes 47 seconds on Google+ in March vs. 6 hours, 44 minutes on Facebook according to figures Nielsen supplied to Mashable. However, that number is down for Facebook. In March 2012, the average was 7 hours, 9 minutes per person. For Google, the figures are a substantial jump over the 3.3 minutes users spent on average on the site in February 2012, according to comScore.

Elsewhere, Nielsen reports that 20 million unique visitors in the U.S. used Google+'s Android and iPhone apps, a 238% rise over March 2012. On desktop, G+'s monthly uniques jumped 63% vs. the year before to 28 million. Nielsen didn't supply an aggregate figure that included desktop and mobile traffic to the network. There is likely a lot of overlap since visitors access the networks both via desktop and mobile.

The figures compare to 142.1 million uniques for Facebook's desktop site during the same time and 99 million uniques who visited Facebook via their mobile devices. Twitter had 34 million unique visitors on desktop and 29 million uniques visiting from mobile.