Share 'Does Twitter really help with book sales?'

Ever since I became published, I keep being told to use "Twitter". Why? Yes, it's another form of social networking, but does Tweeting actually produce sales?  I already maintain a good number of social forums, how can adding Twitter help?   I'd really like to know and see cold hard facts before adding another social outlet to my daily schedule and take more social time away from writing.   Thank…