A common answer to 'why Twitter, it's not a mass market tool?', is 'because the people who do use it matter.' The influence over numbers argument.
That's backed up by an ExactTarget study, reproduced in emarketer. US Twitter users in April 2010 were far more likely than general Internet users to post to forums (75% vs 25%), blog (72% vs 14%), comment on blogs (70% vs 23%) and post ratings / reviews (61% vs 20%).
In other words, the 14 million odd people who regularly go on Twitter (as opposed to the 95 million that have signed up), are already active in social media, know how to make things happen and to create noise - good or bad - online, and take conversations elsewhere, be that to blogs, forums, other social networks, or even the mainstream media.
As Morgan Stewart of ExactTarget puts it, "What happens on Twitter doesn't stay on Twitter. While the number of active Twitter users is less than Facebook or email, the concentration of highly engaged and influential content creators is unrivaled-it's become the gathering place for content creators whose influence spills over into every other corner of the internet."
ExactTarget also asked Twitter users why they followed brands. The biggest reason was for informational purposes, with updates on future brands (38%) and keeping informed about the company (32%) being the main reasons.