I know you posted this a while ago and in the meantime probably everyone is on twitter, but I just got here so....!
I use twitter mostly for business networking, but not for selling, rather for building contacts, both people who might be customers and people who might advise me, inform me, supply me or support me when I am having a rotten day.
I wasn't sure whether it was a colossal waste of time but have learned to use it more effectively and it is now a major source of new contacts. Infact my largest source.
I watched a brilliant free training video about using social media by a guy called Perry Belcher which was amazingly useful - it basically answered my main question about twitter etc which was "why would anyone buy from me on social media? and therefore am I just wasting my time?" To which the answer is "don't sell on social media, build relationships, do the selling elsewhere."
Anyway if you are interested the link to the video is at
http://socialmediamoneysystem.com/blog/?p=1
Hope it's useful!