If you already have a Facebook page, Twitter mailing list profile or YouTube account, then you have a rich pool of targeted traffic. People only want to do business with those they know, like, and trust. With social marketing, you are miles ahead of the curve. I personally know someone who is earning more than $25,000 per month from her Facebook page.
She's built her consulting business one client at a time using the most popular social network in the world. The trick is that there is a fine line between the "social" aspect and the "marketing" side of social marketing. If you lean too much mailing list on the social, you'll never make any sales. Lean too heavily on the marketing, and you'll turn off your friends and colleagues, even in a business-oriented network such as LinkedIn.
Not everyone agrees on how much is too much, but I recommend that only one in 4 or 5 posts should be commercially oriented. And that's mailing list pushing the envelope. These should be sincere product recommendations from one friend to another - not blatant selling. People don't like to be sold to, but they do love to buy.