Getting the community talking about you is in many ways free SEO. If iJustine links to you in her twitter feed, 5000 people may show up in the next 5 minutes. That is a very powerful medium. Sure that is less than what shows up if you do TV ads, but iJustine will get those people talking about your product, and lend credibility to your site in a way that a TV ad can’t.
That isn’t to say you shouldn’t do TV ads (Blackwater Ops offers TV ad optimization as well). But it is important to know what message should be put in which place. With Social Media your dialog is two-way, so seeding the discussions with people who will evangelize your product is important. When you do traditional advertising you broadcast a message, so you can be certain that the message is what you want, with social media if done correctly the message can be what your audience wants.
For instance: If you are Nissan you can do a mass market ad about the ride and performance of your cars, but in the social media space you can truly “touch” the enthusiasts by talking about the aftermarket, and how the GTR got a laptime faster than the Porsche Carerra on the Top Gear test track.
And if you work this magic in the forums on the Top Gear website you build core followers who become evangelists for your product as well.