Because Big Companies Are So Scared of Social Media.
Social Media is about, you know, socializing. Engaging back and forth with someone. It's not the same as a TV or Radio ad. Your audience isn't passive, meaning that neither should you be.
For small companies, social media affords them the rare chance to actively engage with their consumers. To find out what customers want, what factors affect their decisions, and how a business can provide more value.
The actual article linked to above mentions niche sites, (i.e., the Long Tail) that get more hits in a week than corporate sites get in a decade. The truth is that most corporate sites suck, because for most of them, they're useless. Why would I ever go to the Pepsi website? The only reason I might consider would be to nab one of those crazy insane flavours they sometimes make (take a hint, Pepsi). So big brands like Pepsi set up viral marketing campaigns, which sometimes are quite effective, as in the case of Pepsi + Mentos Youtube videos, and sometimes fall flat (the hundreds of other corporate viral marketing campaigns you've never heard of).
Now, for a small site, they can probably provide value online. A small restaurant can post its menu or driving directions. They can even add a reviews section to their menu. Or even a recipes section for a few of their popular dishes. Freelancers have it even better. They can just add clients to Facebook or Twitter and actually interact with them like a human being.
I say take advantage of web 2.0 if you can. Just don't try and disguise an ad as basic interaction with your customer* - they're not all dumb and you'll look kind of silly.
*Note: Actually advertising on social networks is fine. Just don't pretend like that's not what you're doing.