I’ve seen lots of discussion for and against link shorteners, but not specifically this line of argument:
Let me grab a random shortened link from Twitter. Don’t go away, I’ll be right back.
OK, that’s six characters in the domain, a slash, and six more characters. 50 years from now, if bit.ly is still in operation, the URLspace will be rather more crowded, and the part after the slash might be eight or nine characters. This is a significant cliff, since most people have trouble remembering more than 6 or 7 things in their head at a time. Thus, one could conclude that 50 years from now, newly minted bit.ly URLs will be less fashionable than those from newer link-shortening services, particularly if more short TLDs come online, which seems likely. In that scenario, fewer and fewer people will use bit.ly, and it will become a resource-pit as costs go up (for more database storage, among other things) while usage drops, an economic trend that has only one eventual outcome, leading to the breaking all the external links relying on this service.
I’ve been picking on bit.ly here, but the same principle applies to any shortener service. In fact, the more popular, the more quickly the URLspace will fill.
The moral: don’t use link shorteners for anything that needs to be more durable than something you’d scribble on a scrap of paper at your desk.
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