Twitter activity at all-time low, only 17% of users actually tweet
The delicious stats go on! About 80% of Twitter users have tweeted less than ten times in total. 40% of all 75 million accounts have never tweeted. 25% have zero followers. Also, while the service is still growing (6.2 million new accounts in December), it seems its growth has peaked and is now beginning to fall: July saw 7.8 million new accounts. The growth graph still looks very impressive -- it's just a matter of the inherent quality of those new users.
The report ends on a good note (for Twitter users at least): it doesn't look like its loyal users are even thinking about jumping ship. Basically, despite the 83% of users not tweeting on a monthly basis, the remaining 17% make up them in a big way -- they're highly-loyal to their followers and actually become more active over time.