I rarely use Twitter, but when I do it is usually to tweet my distaste for a certain company. Whether I am the victim of bad customer service or a business just does something that I think is wrong, I tweet about it.
Sure, I'm limited to a certain amount of characters, and when all is said and done, nothing is resolved. But I get a kick out of how fast the company responds because they know bad news spreads fast - especially when bad news is posted on social networking sites.
Of course, I don't limit my complaints to just Twitter. I take advantage of surveys that company's put on their receipts, "contact us" emails company's post on their websites and so forth. It's a great way for me to vent my frustrations without having to actually talk to someone. And, honestly, I usually always get a response back and more often than not I am sent coupons for free stuff as an apology.
There really isn't any excuse for poor customer service these days, so if you're not sending e-mails when you're treated poorly be sure to start! You'll be surprised what company's will do to keep your business.