[Retr0spectrum]

You've shortened 6,463,545 links so far. Assuming you're using a charset of [A-Za-z0-9], that gives you 14,776,336 total possible 4-character URLs. Almost half way there!

What's your plan once you reach the 4-character limit? Roll over to 5, or something fancier?

[TimLeland]

Yes, you are correct. I don't have anything fancy planned. Users with accounts are able to customize the ending so this will increase the availability. Currently, Bitly is up to 7 characters long when you create a random short URL. T.LY is already 2 characters shorter and there are still plenty of short random URLs 62^5 = 916,132,832

[sethammons]

years ago, we asked bit.ly if they could vendor our link shortening and tracking. Turned out we were already a couple multiples larger than them in terms of link processing. our use case is a bit different though. Instead of one link given to many, we tend to be one link given to one recipient. Optimizing this kind of problem is interesting and fun.