6B a month =~ 2k per second. Just putting the numbers in perspective :-)
Furthermore, these are just click events. It's okay to lose a few, so the design doesn't have to be especially good at making sure events aren't lost, as far as I can tell.
A good design is as much about what is left out as what is left in, which is the lesson between the lines here in my opinion.
This is the equivalent of (as per my example) yelling out "I hear ya! Coming..." and then take 15mins to reach the door. I never said that low latency implies anything about the level of availability; I merely meant that arguing about the availability of systems is incomplete without a thorough discussion of latency.
In the case of Bitly, I'm just curoius about the systems that are highly available but "require" low latency vs systems that don't require it. As ryanjshaw points out, the system may have a degree of tolerance for lossy click events. If you have a heterogeneous mix of systems with different tolerance levels, that surely affects the architecture does it not?