>They've got to recycle the short urls.

you'd think so, wouldn't you? but, from the snipurl FAQ:
How long will the snipped URLs be maintained?
In theory, forever. Snipped URLs never die, they just get spam-protected (if anything).

this is their claim, not mine. I don't know how they can make this claim, nor do they deign to gloss it.

edit
from makeashorterlink.com:
How long are the shorter links going to last?

For as long as possible. Put it this way: as long as we are in charge of the database, the links will continue to work. If the time comes when we run out of money or interest in maintaining it, we shall make the database available to anyone who wants to take it on. We hesitate to say "forever", because that's a very long time indeed. But the links will remain usable for a long time.

it almost reads like a gloss of the snipurl claim.