Facebook growing fast, MySpace still on top.

The good news for Facebook, according to new statistics from Hitwise, is that its traffic is up 50 percent in the U.S. since last August. The not-so-good news for Zuckerberg & pals? The same numbers say that News Corp.’s MySpace still owns a whopping 67.5 percent of the social-networking market in the U.S.

Hitwise gathered its data from an analysis of traffic to 56 different social-networking sites, and concluded that Facebook has gone from a market share of under 14 percent to slightly over 20 percent in the past year. MySpace, meanwhile, has seen a 10 percent decline in visits, which has pulled its share of the sector down from over 75 percent last year. These numbers, however, were tabulated well before this week’s launch of MySpace’s music service, which may well boost its traffic.

Behind Facebook and MySpace, Hitwise found that the third, fourth, and fifth most popular social networks in the U.S. are MyYearbook, Tagged, and the AOL-owned Bebo. None of them, however, has yet to bring in more than 2 percent of the U.S. market share.

Facebook vs. MySpace traffic comparisons are popular among data firms these days, withComScore announcing in June that Facebook had passed MySpace in traffic for the first time. But much of Facebook’s growth is overseas, and everyone seems to be in agreement that MySpace is still the top social network in the U.S.

But Hitwise had some more news that might not be so good for either MySpace or Facebook: Visits to social networks overall were down 17 percent from August 2007 to August 2008.