This chart says it all:
If you’re using anything other than bit.ly to shorten your links – certainly if you’re using TinyURL – you’re actually hurting your brand. For a long time Bit.ly was Twitter’s URL shortener of choice and that recommendation, plus the stats and convenience that bit.ly provides, make it nothing less than essential for those looking to get their content re-shared. And while the bit.ly interface isn’t quite as slick as it used to be, it’s still the only URL shortener you should be using.
Tip: the bit.ly sidebar is super-convenient.