There are many cases where a social site shot itself on the foot and committed suicide by taking one bad move, such as Digg. What you're suggesting sounds like it will set a new record for a bad decision. Let's say Twitter does make that mistake and actually starts charging high end users. It is extremely naive to think most of these people will stick around and pay. Some will start leaving. Some new services will arise that claim to provide the service for free. Some of these "celebrities" will join forces to create a new "free" network (because that's what Web is supposed to be). Overall their userbase will decrease. And just as it grew exponentially after it reached critical mass, the reverse also applies, and at some point it will die at an exponential pace. It will probably end up looking something like MySpace.