The average Facebook user with 500 friends actively follows the news on only 40 (8%) of them, communicates with 20 (4%), and keeps in close touch with about 10 (2%). Those with smaller networks follow even fewer," according to the June 1, 2009 BusinessWeek cover story, What's a Friend Worth?
Do those stats hold true for you? My monitor rate is much higher, I'd guess closer to 20% of my FB friends' updates I follow, but the communication and keeping in close touch rate is about right for me.
Another fascinating dynamic for managing corporate communications in the Web 2.0 world: Managing the Tweets: Companies are scrambling to silence errant messages while exploiting social networks.