The love affair between Internet-connected Americans and social networks continues, with two-thirds claiming to visit sites such as Facebook, Twitter, and MySpace, up about 230% from the 20% penetration in 2007, and 43% are visiting these sites more than once a day, according to a new report by Experian Simmons.

Multiple visits are up 28% compared with last year, the Simmons New Media study determined.

"The rise of social networking tracks closely with that of Facebook. As of April 26, 2010, 46% of the U.S. online adult population reported having visited Facebook in the past 30 days," said the report.

People primarily logon to stay in touch with friends, followed by for fun and to communicate with family, according to the study. Career- and job-related reasons were low on the list: 19% said they used the sites to keep in touch with professional contacts; 9% wanted to make new professional contacts; 6% sought professional development; and 5% said it was part of their job, the report found.

Last week, a study conducted for Cisco found that 68% of IT decision-makers determined employees were accessing social networking sites from work.

"As the lines between personal and business computing increasingly blur, it is becoming clear that employees are going to use social networking and personal devices whether permitted or not," said Fred Kost, director of security solutions at Cisco, in a statement.

Social networking sites' impact on business is growing. This year, 68% have become a fan or friend of a product, service, company, or group on a social networking site, compared with 57% last year, the study said.

Different social networking sites command different loyalty. For example, the top indexing retail sites on Twitter included Nordstrom, Forever 21, GameStop, and Victoria's Secret during one 30-day span. In the same timeframe on MySpace, that list featured Hot Topic, Foot Locker, Anchor Blue, and GameStop, while Facebook's top-ranked retailers consisted of H&M, Hot Topic, Forever 21, and American Apparel.

Social networks are not only for the young: While almost 90% of connected 18 to 34-year-olds visit these sites today, 41% of online adults aged 50-plus make monthly visits to social networks, the study said.