MySpace still holds the largest market share for U.S. visits to social networking sites, while its competitors show much stronger year-over-year market share increases, according to a survey released Wednesday by Hitwise.

MySpace.com's share of U.S. visits fell 6% from June 2007 to June 2008, Hitwise said in its report. At the same time, MyYearbook increased its market share by 318%. Still, that site only holds 1.54% of the total market share for U.S. visits and ranks third among social networking sites, Hitwise found. MySpace has retained its ranking as first among social networking sites for U.S. visits, with 71.92% of the market share.

Facebook ranks second, with 40% year-over-year growth and 16.91% of the market share for U.S. visits, while Tagged increased its market share by 45% year-over-year to 1.08%, according to Hitwise.

Bebo's market share of U.S. visits fell by 41% year-over-year but the site remains in fifth place among all social networking sites in a Hitwise list of the top 57, behind Tagged.

U.S. visits to 57 social networking sites monitored by Hitwise fell 19% from June 2007 to June 2008, while rising 7% from May to June of this year. That reflects an uptick in activity during the summer, which has occurred for the third year in a row.

Users spent an average of 27 minutes and 34 seconds on social networking sites in June 2008, up 9% from June 2007, Hitwise found. MySpace users spent an average of 31 minutes and 12 seconds on the Web site, while Facebook users spent an average of 21 minutes and 6 seconds on that site last month, Hitwise said. That's an increase of 41% since June 2007.