You're joking, right?
If that's not mainstream, I have no clue what is.
Politicians everywhere have learned to adopt technology early, it's critical for public relations and community-building. Just like Obama visits Facebook or Google for a Q&A. So that alone, and even the niche who follow every tweet (as opposed to the vast majority who pick it up second or third hand via journalists) is not mainstream.
Mainstream is hundreds of millions of ordinary people sharing photos with each other. That's the game Google and Facebook are playing.
Twitter is mainstream. Besides celebrities and other public figures, it's a way for people within industries to follow one another.