[willscott]

This is exactly what happened with the first era of URL shorteners, and has the detrimental effect of scattering dead links around the web when your service closes. (termed link rot)

ArchiveTeam has a project scraping URL shorterners to attempt to archive exactly these things. Their tagline is "url shortening was a fucking awful idea".

http://archiveteam.org/index.php?title=URLTeam

My takeaway from this is that having a whitelist is fine, but keep the original URL visible so that if your service isn't available the original content is still visible (although potentially requiring manual user effort) is a good thing.