From the WSJ:

Twitter is having no trouble signing up users. But some new research provides an update on the size of an ongoing problem: getting people to tweet.

A report from Twopcharts, a website that monitors Twitter account activity, states that about 44% of the 974 million existing Twitter accounts have never sent a tweet.
I think Twitter is an extremely powerful platform, but I find there's a lot of noise on it—even when I follow smart, thoughtful people. Interesting links and thoughts are interspersed with trying-to-be-witty one-liners and jokes.
The other aspect I find annoying as fuck about Twitter is how mainstream media have taken it over (one of the more recent reasons I don't watch much television). The barrage of "follow us" and "use hashtag: #ilovecloroxbleach" really get on my nerves.