I had a feeling that at least some of social networking's growth had to do with parents joining to learn about this huge presence in their kids' lives. Now the Pew Internet Project has some data at least on grownups, if not parents. "The share of adult internet users who have a profile on a social-networking site has more than quadrupled in the past four years – from 8% in 2005 to 35% now," Pew reports. A few interesting observations from Social Computing Magazine's coverage of the Pew study: Like teens, adults use social sites to communicate with people they already know (89%), and most have profiles in multiple sites (51%). Among US users 18+, MySpace is twice as popular as Facebook (50% compared to 22%). Here's the breakdown by age on who has social-network profiles: 75% of people 18-24; 57% 25-34; 30% 35-44; 19% 45-54; 10% 55-64; 7% 65+. Here, too, is Business Week's coverage, and the Washington Post talks about job-hunting with social sites.