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 remain active and will continue to point to their destinations.
You can still use the bit.ly domain to shorten your links on your account. On most paid plans, you can also create branded links using a custom domain.