Nambu Networks says it attributes Twitter with the success of competitor bit.ly, calling it the "clear market winner." It went on to say that Twitter has all but sapped them of the energy to double-down and further develop tr.im. In addition to the fact that bit.ly is the default for Twitter users, tr.im recently suffered from a denial of service attack last week which knocked it offline and certainly didn't help anything.
Tri.im is no longer accepting new links, and existing URLs will be redirected until at least the end of this year. There has been talk of the service living on as some form of an open-source project, but nothing is certain at this point.