As someone who manages numerous Twitter accounts, both for my own businesses and for my clients, I found these recent statistics from eMarketer quite interesting. It seems like almost everyone (92%) is aware of Twitter but a much smaller percentage (13%) have used it.
Of course, it's important to keep in mind that the usage number has grown by more than 62% in the past year.
In managing client Twitter accounts, I've found that it has become a very rich source for connecting with "active influencers" such as trade publication editors, industry analysts, etc. as well as with prospects and customers.
With some targeted up front work and a commitment to on-going activity, it's now pretty easy to quickly build a quality following/follower list and to become a recognized participant in specific communities of interest.
While broad-based Twitter usage still has a relatively long way to go, it has already become a very useful tool in the overall bag of business-to-business marketing methods.