The rise is driven by all ethnic and age groups, but women stand out as the most avid users, according to the Pew Internet & American Life Project.
The study reported that 65 percent of all online adults surveyed in May said they were using social networking sites, up from 61 percent a year ago. The social media users represent 50 percent of all American adults, Pew said.
On a given day, only e-mail and search engines are used more than social networks by adult Internet users, Pew said.
“Social networking sites continue to cement their place as a significant part of mainstream online life,” said Kathryn Zickuhr, a Pew research specialist and co-author of the report.
Seven out of 10 women said they used the social media sites, compared to six in 10 men. Young women were particularly active; nine out of 10 ages 18 to 29 log onto the sites.
Indeed, women have become a focus of marketing agencies that are trying to appeal to online moms — a particularly active group of smartphone users. These mothers are sharing recommendations for products, saving pictures of children and family, and using social networking sites to keep in touch with friends and relatives.
Social networking sites have come under closer watch by federal regulators pushing for greater privacy protections for consumer data. Regulators have proposed a Do Not Track mechanism to allow users to keep Web sites and marketers from following them online. Facebook, Google and other Web giants have faced investigations by the Federal Trade Commission over privacy violations.