I think part of the issue is that Twitter is a great tool for people in media/news as well as PR/celebrities etc, and because of the role these groups play in s

I think part of the issue is that Twitter is a great tool for people in media/news as well as PR/celebrities etc, and because of the role these groups play in society they have been doing what amounts to a lot of free advertisement for Twitter. All along this may have been "artificially" increasing Twitter's user numbers relative to the kinds of users they want.