The number of US adults who use social network sites has actually more than doubled since 2007, Mashable reports, citing a new Forrester Research study. Forrester found that just under a third of adults, or 55.6 million people, visit social sites at least monthly, up from 15% in 2007 and about 18% last year. Video viewing, shopping, and email are still more popular than social networking, but SN growth is steady. That, watching/streaming online video, and listening to/streaming online audio are the only three of ten Net activities that show steady growth over the last three years in a Forrester chart. You’ll find more Mashable social-media numbers here.