Url's shortening services provides a way to shorten those long urls in webpages, making less awful to the "style" of the page. Imagine someone, post 4 urls, 2 lines per url? Shortening Services make more friendly those weird (with hashes or shit) urls.
But... there's always a but!, you never see the real url where the browser will be pointed to. So, here is the risk of been forwarded to a page with malicious code within (malwares, trojans other malicious routines).
A solution to this problem is to unhide/reveal the real url, you can
install a firefox plugin (Long Url Please version 0.4.2).