If that Pew Research survey is there only source (it's the one they link when they make the claim) it looks like 25% of Americans use twitter. But that doesn't mean that 25% actually makes tweets. It's probably possible to cross that with public twitter numbers on active tweeters, but I've heard 1% make 99% of the tweets before, and in my own experience that seems accurate.

Most people I know who use twitter use it to stay up to date with creators they care about.