The number of visitors to the top 10 social-networking sites soared in April, attracting nearly half of all Web users, a market research firm says.

The top 10 sites collectively grew 47 percent in the United States from the same month a year ago to 68.8 million unique visitors, Nielsen/NetRatings said. The sites reached 45 percent of active Web users.

MySpace, owned by News Corp. and a favorite among teens and young adults, topped the list with a year-over-year growth rate of 367 percent to 38.4 million unique users. Blogger, owned by Google Inc., was second with 18.5 million visitors and an 80 percent growth rate.

Classmates Online grew 10 percent to 12.9 million visitors, and YouTube and Microsoft's MSN Groups, which saw a 14 percent drop in visitors, rounded out the top five with 12.5 million and 10.6 million, respectively.

Jon Gibs, senior director of Nielsen/NetRatings, said social-networking sites are the "reality television of the Internet."

“The content is relatively inexpensive for publishers to produce, and social networking is not a fad that will disappear," Gibs said in a statement released this week. "If anything, it will become more ingrained in mainstream sites, just as reality TV programming has become ubiquitous in network programming.”