WSJ has a great article with examples of companies who tackled Tweets and social networking by themselves and some who used an agency. This article lists some of the monthly retainers paid and a list of firms who do this type of work. I found it very insightful especially since I have a few clients who are considering a jump into the social networking world–in a big way.
I find that many companies have just established a presence and let it sit. They feel that simply posting information will draw an audience to you. It takes much more effort than that. While the upfront investment in time can be substantial, there eventually comes a time when the process becomes automatic. Some of the new social media press release sites allow you to set up the Tweet message so anyone who views your press release can easily circulate it many ways: Twitter, Facebook, LinkedIn, del.icio.us, etc.)
I recommend reading two recent articles from the Wall Street Journal: the one posted here: Firms Get a Hand with Twitter, Facebook and A Start-ups Tale: Tweet by Tweet . Remember, these links are only valid for seven days so download now.
My other advice, subscribe to the Wall Street Journal!