At one time, Bitly offered j.mp and bitly.com as generic domains to shorten links. These domains are no longer available. Previous short links created with these domains cannot be redirected.
Existing links on j.mp and bitly.com will continue to decode to their destinations and will remain active.
If you don't have a Basic plan with your own custom domain, you can still use the bit.ly domain to shorten your links on your account.