I can't quite tell what the tone is of people writing about teens and their use of the Internet today (and what I mean by that is, really mostly teenager's shunning of Facebook). That tone seems to range somewhere between admiring fascination and panic: the tones that people adopt when studying a group of people who are so incredibly different from us. But they're not actually that different.
I think one of the big disconnects between teenagers and the generation building products and services for the Internet right now is that teens use Facebook almost purely as a utility: to keep idle track of people they've met or to send messages and maybe organize events. But the idea that one would “hang out” on Facebook as you would, say, Tumblr, is as ridiculous as the idea that one would “hang out” in your email inbox. Yes, we interact with friends on all three of these tools, and personal messages come through all of them, but they're not all entertainment.
This isn't actually so odd. During one of my first set of needfinding interviews with people about the web, I was looking into how women roughly aged 30-40 used Facebook. The single biggest predictive factor of how someone used Facebook was not how old you were, or whether you had kids, or even how many friends you had. It was how long you had had a Facebook account. Women who had Facebook for less than two years tended to be fairly obsessed with it. Women who had Facebook for more than two years were pretty over it: after a while, the novelty of knowing what all your friends were doing constantly wore pretty thin. It was useful when you needed it, but not so useful otherwise. Similarly, a generation who grew up with Facebook isn't that impressed with it.
What I'm seeing instead is a generation that's struggling to grow up around technology, and especially choice. I think at least a few of them are beginning to understand the nature of this choice: it's not just what they use, but the choice of what goals they use the technology to achieve. The web in the last ten years has become “social,” and just like who you hang out with defines who you are at that age, so does how you use the social web. Some tools are escapist, letting you be a completely different person. But Facebook is this weird extension of “IRL” into the virtual world, allowing hallway chatter and posturing to follow you home. Adolescence (and/or high school) has a funny way of producing more people who feel like they don't fit in than people who do, and so it makes sense that teens are turning to services other than Facebook: services that help them fit in a bit easier.