I won't say it's solely what my company lives off of but it definitely provides X,XXX,XXX in sales more a year. We provide a top fan site in a niche with several mimic websites that all promote our sales on and we generate several hundred usually large sales off of each sale through the Facebook alone. It's definitely possible and it doesn't take as much effort as most people would think. The facebook only has 400,000 followers with around 2-3 million impressions a week.