While Twitter is still gaining traction with most folks, a recent study tell us:
"The average (median) lifetime number of tweets for a single user is 1, while 10% of the users contribute 90% of the traffic."
Perhaps not a great sign of longevity? One tool I've started to use to cull my numbers so I don't appear to be OVERfollowing is Twitter Karma. Head here to check it out: http://dossy.org/twitter/karma/
However, I will lodge a complaint- not all activity is current. I was looking to lower my follower numbers and it indicated that someone had not tweeted in over a year. Yet, go to their account and it was updated hours ago.
Otherwise, it is a good way to visualize what is going on with your followers. Although, as one review points out, only good if you have less than 10000 followers.