Only one in four registered twitter users is tweeting in any given month

"We know that Twitter has 50 million registered users, but we also know that the vast majority of them have tweeted fewer than ten times. Let's investigate just how many of these registered users are actually actively tweeting.
Using our tweet data, we can identify what percent of the user base sent out at least one tweet in any given month.
[...]
The number seems to hover in the 25% range. In other words, only about 1 in 4 registered users is actually tweeting in any given month. (Although it's worth noting that some users may only be using Twitter to read others' tweets, meaning they are not full-fledged "zombie" accounts.)"
Source: Robert J Moore of RJMetrics, writing in Techcrunch, 5th October 2009, using analysis of 85,000 users and 3 million tweets.
Other stats quoted:
"Twitter's user growth is no longer accelerating. The rate of new user acquisition has plateaued at around 8 million per month.
Over 14% of users don't have a single follower, and over 75% of users have 10 or fewer followers.
38% of users have never sent a single tweet, and over 75% of users have sent fewer than 10 tweets.
Once a user has tweeted once, there is a 65% chance that they will tweet again. After that second tweet, however, the chance of a third tweet goes up to 81%.
If someone is still tweeting in their second week as a user, it is extremely likely that they will remain on Twitter as a long-term user.
Users who joined in more recent months are less likely to stop using the service and more likely to tweet more often than users from the past."