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Perhaps it's the siren call of summer, but the sizzling growth of social media sites in 2006 showed signs of cooling in June.

Both ComScore Media Metrix and Nielsen//NetRatings (owned by Adweek parent VNU) reported U.S. visits to sites like MySpace and Facebook trailed off in June. Visitors to MySpace grew less than 2 percent from the previous month to 52.3 million, after averaging 7 percent monthly growth this year, per ComScore. Facebook, which is heavily dependent on college users, saw its visitors drop 2 percent in June to 13.8 million, after an average monthly growth of 3 percent.

Nielsen//NetRatings shows some slowing, although not as uniformly as ComScore. According to it, MySpace gained 9 percent more visitors in June from May, after adding 10 percent a month this year. Facebook's audience grew 3 percent in June, after a 10 percent average monthly gain in 2006, per Nielsen//NetRatings.

A slowdown in the growth for social networks, particularly MySpace, would come after a massive expansion in the past year. MySpace now has 93 million members, adding nearly half this year.

Smaller social networks also suffered summer doldrums, according to ComScore. MSN Spaces lost 8 percent of its users in June from the previous month. Tagged's audience fell 30 percent. Bebo saw visitors drop 2 percent.

The tapered growth might be an aberration, warned a ComScore rep. In a sign of how far social networks have come, Hitwise, an Internet measurement firm that tallies total visits rather than visitors, said last week MySpace is now the most visited site in the U.S. Hitwise's figures show social networking sites now making up just under 5 percent of all site visits, a 39 percent uptick from the start of the year. "What we're seeing with social-networking sites is they're becoming a home base for consumers, much like the portals were in the 1990s," said Bill Tancer, a Hitwise analyst.