One of the points I always make in explaining the potential dangers of so many online social sites for young people is that most of these sites were designed for a broad market and for many purposes. Hence, being upset because such sites contain content or offer the potential of inappropriate experiences for young people cannot be blamed on the site. You should know what you might encounter.
This said, these sites are attractive to young people for the same reasons they are attractive to adults. The sites offer interesting social opportunities.
This post and “infographic” from ReadWriteWeb offers some data on “under age” use of social sites.