I'm going out on a limb: Twitter may still be rising, but before long it'll be on the way out. By Jan 1st, 2011, Twitter will have fewer active users than it does today. Maybe even sooner than that.

Why? Because of the main findings of a surprisingly insightful guest post on Techcrunch by Geoff Cook, CEO of myYearbook. He pulls together the best available evidence on the question of why "Teens don't Tweet", and combines it with his own survey data. The nutshell is: Teens don't tweet for the same reason that most people don't tweet. Twitter doesn't provide much that other services like Facebook and MySpace don't already provide.

Twitter was itself pretty much a clone of earlier services such as Jaiku, recently acquired by Google, then open sourced. When people ask me what Twitter is, I say "Well, it's like Facebook status updates, but without all the other stuff on Facebook." Twitter won't last because its value proposition isn't large enough to sustain a user base beyond the initial fad-driven period. And that period is almost over. Yes, there really is a segment of the market that is hip-deep in micro-blogging, constant sharing. And I think Twitter will live on as an aggregator for news and updates from companies, celebrities, news outlets, and the like. But as a broad-market tool, it's 15-minutes are almost up.

I'm not even sure that Twitter sees Facebook as a direct competitor at the moment, but it ought to, because it's getting out-innovated by them left and right. Facebook's new iPhone app. is fantastic. They're expanding features and APIs, becoming a platform for gaming, for social organizing, for direct communication. Facebook Connect is getting traction, and threading Facebook into a whole network of external sites.

Facebook will survive for the same reason Wikipedia does: it's rich and diverse enough to foster a whole ecosystem around it. That's *very* hard to do, but once you've got that kind of diversity, you've got staying power. In the academic world, Facebook and Wikipedia are called public goods. In many cases, with public goods we're looking for the relatively few individuals who have so many resources, and who benefit disproportionally from the provision of the good, so much so that they're willing to provide the good on everyone's behalf. We called these people 'privileged groups.' The reason that diversity makes Wikipedia and Facebook so stable is that they don't attract just one privileged group, they attract many. There are so many ways to benefit, so many ways to engage, and for each one there's a privileged group. The interests of these groups overlap and enforce each other, together synthesizing a product that's much more valuable. Twitter just can't compete with that.