Seems like there are some changes coming up on twitter too. Earlier twitter went with Tinyurl - a default URLs shortening service for it. But now bit.ly has replaced its place as the default URLs shortening service. Twitter haven't given any reason behind this change so far.
However unofficial reviews on web behind this change suggests that Betaworks, the startup behind Twitter related companies such as Summize is also behind this bit.ly, and this move from Twitter promps that it is making the way for itself for the acquisition of this URL shortening service provider.
Bit.ly has come a competing service among the URL shortening services which has raised $2million from several angel investors