Social Networking Generating leads and closing sales for Marketers...from MediaPost "Research Brief"

88% of marketers polled at the social media summit 2009 are using some form of social media to market their business, though 72% of those using it say they have only been at it a few months or less. 

Source: http://www.mediapost.com/publications/?fa=Articles.showArticle&art_aid=104386

At Zig we are testing and using all both for clients as well as to generate leads for our own company.  We have been having great success implementing Twitter for both clients as well as internally.