There is an interesting article on Mashable today about a Harvard Business School study (http://bit.ly/1Gs40W) done on Twitter users. (Side note: Is this what they do studies on at B-school?? Um, why do I not go there...)
Key Finding: 90% of tweets are posted by 10% of twitter users. For comparison sake, Based on eyeballing the graph, it looks like Facebook is around 90% of use from 55% of users.

More shockingly - The median number of lifetime tweets per user is 1. Of course, most people's lifetime is only a few months. But the point is a very small number of users are doing all the work.
So what does that mean? Most twitter users are using twitter to listen. Not talk. It is a place to collect information, easily digestible updates on things you are interested in. This can include friends, industry trends, wine. In some sense an analysis of the people follow could you a pretty good sense of their top interests.
What does this mean for brands? I think it is good news - It means a good place to talk to your consumers. Assuming of course you have something to say. It is also a good medium to easily identify your most engaged (good or bad) consumers. In terms of how to use it - don't expect a lot of collaboration or RT (retweeting for the non-twitters out there) from your audience and take that into account when developing your strategy.
All that aside, if you take nothing away but this I will be happy: Social Networks are different there is likely no such thing as a one size fits all approach to social media. Not every brand has to be on Facebook, Twitter, Myspace & etc. Pick what works for you and develop your strategy appropriately.