A friend of mine who got on the featured users list and went from 10,000 followers to 1,000,000 followers said it was like nothing changed. The number of @replies and RTs didn't increase noticeably at all.
The first time I have ever heard of Justin Bieber was by seeing his name in the trending topics for weeks in a row. (first half of the year) So a lot of Bieber fans do tweet.
I think twitter changed their trending topics algorithm to not show Justin Bieber every time, they opted for new popular topics instead of always popular topics.
Twitter has always been more of a 'broadcast to followers' than a conversational tool.