Do you have to pick just one? If not I'd throw in Yahoo!Groups and even MySpace and Craigslist. It all depends where your target users hang out most. In my personal experience a combination of several worked best.
They are all different. You should do a combination of those to see which work best for you. Depending on your industry and target audience, some may be better than others. Also see:
Speaking intuitively, to me it seems google comes last among the three. The cons of google are that its a very big jungle with lots of uncertainties and competition with other marketers. I would go with following:
But then again its very subjective and based on targeted customers. Personally, IMO, Twitter is best for starters, while google is loosing its sheen on emergence of twitter and facebook.
As an example this website on personal development, a start-up, is of my friend and he is primarily and effectively, even though he's a newbie, using twitter for marketing.