25% of US Web time is spent on social networks

15 minutes of every hour we spend online is devoted to social media, according to the latest research from Nielsen.

The firm found that Facebook was far and away the most popular social networking site, and thus the most popular site period. That shouldn’t come as a surprise to anyone.

What may come as a surprise, though, is Tumblr. Nielsen specifically called out Tumblr as “an emerging player in social media.” The online tool allows users to post micro blogs and status updates, as well as share pictures and video.

Tumblr’s user base has tripled since 2010 and while that isn’t saying much since it wasn’t a huge player last year, Nielsen thinks it shows a lot of potential for continued growth.

Regardless, Facebook was the highlight of the study. With 750 million registered users, the online steam train just cannot be stopped.

Nielsen reported that in May, US Web users spent 53 billion minutes on Facebook, significantly more than any other site on the Web. By comparison, users spent a total of 12.5 billion minutes on Google.

The average Facebook user has 130 friends and clearly enjoys sharing a lot of time and content with those friends. There’s no end in sight for the explosive growth that the social network continues to show.

Even as we reach the point where Facebook can only grow so much more because there are only so many people in the world, it continues to go forward at full steam ahead. The site is expected to file for an initial public offering in 2012, which could be the most important IPO to happen since Google.