While Facebook has its own URL shortener for mobile links, fb.me, it also works with third-party shortening services, like Bit.ly — that service provides a variety of shortened URLs, as well as analytics and other features. This allows users and Page owners to share their own, custom URLs in status updates and other links.
Why? The reason doesn’t appear to be competitive, as expected. Facebook says:
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.
Facebook has occasionally blocked third parties due to these sorts of issues, and as long as spammers and other ill-intentioned actors use URL shorteners to go after users, we expect it to sometimes do so in the future.