If people's bit-sized thoughts are extremely valuable, why haven't anyone managed to really monetize either IM status messages or the somewhat longer thoughts that are blog posts? Or the other way around, why haven't advertisers seen a ROI gap a'la search had in 2002-2003 and started to jump onto the opportunity? (Maybe a great revenue model to-be-implemented is what Twitter has shown to investors, but I don't see real-time search of some of the world's current thinking being the next great model).
On a related note, according to Comscore, Facebook added 21 million unique visitors between November and December, compared to Twitter's 4.3 million unique in total. And between December and January it seems like Facebook's U.S. new members growth was an insane 60 % from an already high level. If one believes the numbers, the real action currently is with Facebook and not with Twitter.
Update April 26, 2009: Twitter usage exploded in February and March growing crica 100 % per month to 19+ million unique visitors in March, so now the real action is with Twitter as well.