It's actually just a part of the code that we forked from, in particular something called an fnid. Hubski's codebase is forked from hacker new's code and we've basically just hacked on top of that. Expiring links was their solution to having user-specific actions such as submitting posts. When they started out they didn't have many actions that needed fnids but since we did we just added them as necessary. If you look at HN's links now you'll see that for a lot of things they've changed to use simple endpoints with query strings. We're going to be moving in that direction soon, but it'll just take time. Everyone on the hubski team has full-time jobs and family so it probably won't come as quick as we'd like, but it's definitely something we're working on.