I think they are mostly young adults (age groups 13-35 seems most common) and it's mostly loose networks based around geography and education. So, they sort of

I think they are mostly young adults (age groups 13-35 seems most common) and it's mostly loose networks based around geography and education. So, they sort of lie on top of existing real world networks. If you don't have an "in" to Twitter from a friend in real life, you'll find it hard to locate a Twitter network. But once you're in, you can pretty easily seek out new people to interact with, and you can grow your little network and make friends and share cultural capital or whatever.