Internet users in the U.S. are spending more time than ever on social networking services, according to a report by comScore. In 2007, social networking services accounted for 1 out of every 12 minutes spent online — they now account for 1 in 6 minutes spent online (almost 16 percent) .

This drastic shift can largely be attributed to Facebook. The social network is now the 4th largest Web property in the U.S., with 157 million monthly visitors. While we reported earlier this week that Facebook has lost some users, comScore suggests that Facebook cannot continue to grow forever and that it reached 73 percent of total Internet users in the U.S (or 157.2 million visitors) in May. It was also discovered that Facebook users are now spending 6.3 hours on the social network per month, up from 4.6 hours a year ago.

Myspace, on the other hand, is continuing to see a dip among its user base in the U.S. Though it is still in second place among social networks (with 34.9 million visitors in May), its audience has dropped by almost 50 percent in the last year and average user engagement has dropped by 85 percent.

Myspace could soon lose its runner-up position, if comScore’s U.S. data for May is anything to go by. LinkedIn (33.4 million visitors), Twitter (27 million visitors) and Tumblr (10.7 million visitors) have reached all-time audience highs on their websites.

LinkedIn has been growing over the last year, having hit U.S. audience highs in 7 of the last 12 months — it has gown 58 percent in total over the last year. Twitter, meanwhile, has grown 13 percent over the last year to grab 27 million U.S. visitors in May. While these figures only relate to the websites of each social network and many Twitter users tweet through official apps and third-party clients, previous comScore research showed that 85-90 percent of Twitter users visit the social network’s website every month.

Tumblr, meanwhile, is undergoing remarkable growth, having seen a 166 percent increase in visitors over the last year to 10.7 million visitors in May — the first time it has crossed 10 million visitors. This news comes as Tumblr has surpassed WordPress.com when it comes to the number of blogs that each service hosts.

While it has been evident for some time that social networking services are becoming more and more popular, these stats really underline that fact.