Amazing growth in Twitter over the last year makes it the fastest growing online community according to Nielsen's Wire Blog:

"Unique visitors to Twitter increased 1,382 percent year-over-year, from 475,000 unique visitors in February 2008 to 7 million in February 2009, making it the fastest growing site in the Member Communities category for the month."

Also interesting is their note about using social media in an unstable economy:

"It will be interesting to watch the evolution of Twitter as it continues to gain momentum. In an unstable economy, it might prove to be an economical and important part of an employer’s marketing strategy that helps to keep consumers aware of and connected to their brand."