Tons of people every day use bit.ly to provide nice and short links to webpages that have obnoxiously long original links. Bit.ly also provides j.mp which is very similar, but Facebook is allowing none of these links to be used. If you try, you get this popup:
A Facebook spokesperson provided the following explanation to TechCrunch: "As part of our effort to keep Facebook and the people who use our service secure, we closely monitor the content shared on the site for spam and malicious content. At the time we blocked j.mp, more than 70% of j.mp links pointed to spam or other security issues. We are working with bit.ly to resolve the issue." Bit.ly uses several measures to try to avoid spam sites from being used in the service, but there's obviously only so much they can do. For now, if you must shorten site names, just go with the original bit.ly until this is all hashed out.