http://www.readwriteweb.com/archives/is_twitter_really_that_big.php

I
like these two statistics:

*
40 percent of Twitter users have not tweeted since their first day on Twitter
(i.e., the account was most likely created and subsequently forgotten about).

* Approximately 25
percent of Twitter users are not following
anyone, while two-thirds are following less than 10 people (i.e., the
account was created but is not actually being used regularly)