We have something along those lines on the main page of the forum, @
(you need an account to see links). The most popular userbars don't change that much, so the numbers don't really fluctuate in a meaningful way. The same is true for the rarest userbars. A userbar will be rare for the brief time that we've released it, only to fall from the list and be replaced by retired userbars that are only displayed by a small amount of people. In order to mix things up, we limited what shows up on those lists to only userbars we've seen in the past day. Otherwise, certain userbars would stay on that list forever. The most popular userbar of all time never changes because pretty much every person in the forum has unlocked the Pikachu posting participation userbar, and nothing else comes close to topping that.
We could share userbars collected like stamps instead of keeping them walled off like avatars, but what does the community think about this?