I think a large part in that teens already have strong texting networks. One of the reason twitter works is because subscribe-and-publish communication has low

I think a large part in that teens already have strong texting networks. One of the reason twitter works is because subscribe-and-publish communication has low transaction costs per message. Teens engage in signaling behavior that has the high transaction costs of communication as an element of "conspicuous consumption". I have totally no data to back this up, but research into this area is one of my interests, and am looking forward to performing some in the future and seeing if my conjectures hold up.