The share of adult Internet users who have a profile on an online social network has more than quadrupled in the past four years, from 8 percent in 2005 to 35 percent now, according to a new Pew Internet study of adults and social networks. Not surprisingly, use of social networks still remains much heavier among younger adults, with 75 percent of those aged 18-24 using social networks compared to 7 percent of those 65 and older. (Full report in PDF.)