The question now is 'what's next', for twitter, facebook and others, as they reach super-mass adoption, and would (obviously) prefer to avoid 'the myspace phenomenon'. Nielsen already noticed that the 'month-over-month' rate is very minor: +7% in # of users and -1% in time spent - which in social networking terms is bad. That halt in growth has to do in part to the business model, or lack of it.
Twitter is a lot of things, and I salute the guys for providing the platform and building an impressive ecosystem, but at some point in time someone at Twitter HQ will need to step up and present what they plan for twitter in 2010.