URL shorteners do two things. First, they shorten URLs. Second, they create a unique URL to a destination, which can be tracked. That's the key element for marketers. In fact, one of the up-and-comers in this space, bit.ly, has built analytics directly into its interface.
By using URL shorteners, you can begin capturing some excellent insight. For example you can see how many times a particular community forwards a message compared to another community. That is, since you can create two unique short URLs to the same page, put one on Twitter and embed the other in an e-mail. You can then analyze the clicks you get in standard A/B testing.
You can also measure pass-along rates. Since the recipient of the original short URL will most likely pass along the short URL (not the original URL), you can see how far down the particular path you've created to a page goes.
Not Just for Other People's Content and Twitter
The cool thing about URL shorteners is that you can shorten anything. Sure, the traditional way to use these services is to shorten a link to someone else's content and send it via a medium that requires you to be terse. But there's no reason you can't use the same method on links to your own site. Or put it in an e-mail. Or on your own site.
Fact is, URL shorteners provide an easy way to track traffic you're generating via social media channels. Like most things on social media, the cost of entry to use these tools is free. They're definitely worth a try.