When automatically posting to twitter, tumblr uses its own short url scheme (hashing this posts url to something a bit less intelligible). Which is good. There’s been a bit of a blowback against short URLs lately, but twitter needs the shortest URLs it can get. Providing your own canonical short links effectively mitigates most of the criticism about URL shortening services (except for the usable URL argument. But that’s pretty much at odds with using a short URL in the first place so I’m going to ignore it as being petty). FriendFeed does the same thing, but FriendFeed uses the ff.im domain to drive their short URLs. Tumblr just uses tumblr.com. Which, you might notice, is longer.
The summary is this: a tumblr short url takes up 27 characters. A FriendFeed short url takes up 18 characters. And an is.gd url (as far as I know, the gold standard for shortest URLs) takes up 17 characters.
So tumblr is wasting a valuable 10 characters. That’s over 7% of a twitter message.
I will give tumblr some props though: they implement the HTTP link header for short URL autodiscovery. FriendFeed doesn’t implement that or the HTML <link/> variant. So they’re definitely on the right track!