Twitter is most definitely mainstream. When Presidential candidates and major news orgs are using your service, I think we can assume it's "mainstream".
Political activists and major news orgs are among the earlier adopters for most platforms. There are plenty of sports people and celebrities too, and all their watchers. But you could say the same about Second Life a few years ago. Twitter's certainly gone well beyond that, but I still think it's much more of a patchwork of many niches than "the social fabric" that Google Plus and Facebook are heading towards.
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.
There is a difference between adoption and if it is mainstream or not. People knowing what twitter is but not giving a shit about it enough to have an account doesn't prevent it from being mainstream. I'd say Ford is pretty mainstream, but only a patchwork of people drive Ford.
