A year ago, I would have said Twitter, but since that time, Facebook has introduced some better opportunities for marketers to develop their brand there. Some Twitter marketers are finding or developing systems or processes for effective Twitter marketing, but I think it will take much longer for marketing success on Twitter than on Facebook.
Entrepreneurs should use both
as part of their long term social strategy, but I think Facebook offers more immediate potential. (Says the guy who does not yet have a presence on Facebook! lol)