In reality though, most of these kids would abandon Facebook if it cost them to participate – specifically because some other service would evolve that offered them access for free.

Perhaps Facebook could “sell” their service to universities and high schools in order to maintain the availability of this service to their students (for free – at least from the pocket of the student). Once upon a time you had to have an operating email address from a participating educational institution in order to sign up for Facebook – they could leverage this to segment out who should have free access.

But to fully answer your question, can you roll the clock back? No, you cannot simply start charging people for that to which they already have free access and not encounter a significant decline in your network base. The freemium approach requires enhancing what is available to those who sign on and pay the fee so that they perceive it as a net gain rather than a net loss. Recall that Tversky & Kahneman framing psychology discussion – prospect theory – in your marketing courses: for most people losses loom larger than gains.

And the minute you start charging them creates a focal point for decision – how much do they value the service and what are their alternatives? I agree that Facebook offers value, but I could probably accomplish much of the same using some other social media solution that is freely available. But given that my network is built and not easily transported, there would be staying power. Witness how many people stayed with AOL for so long simply because they didn't want to lose their email address and AIM contacts. So my thought is Facebook would likely lose the younger generation unless segmentation was made available, and potentially lose many of the 18 -60 somethings unless greater value in new features were available to premium users. Flickr faced a similar issue and successfully made the transition, though they did it fairly soon after launch (3-6 months post launch if I recall correctly).