HyperLogLog is quite neat, and I very much enjoyed antirez's writeup as well. There are probably a lot of uses for it beyond e.g. counting active users. I'd love to hear about them.

But nearly everyone uses counting users as an example. For this kind of use, I honestly have to ask: at WhatsApp's scale, is 5GB of ram really an issue? It seems like they could probably keep that exact setup and roll it over every minute and not even really tax a modern server.

Or compact it - one bit per person, lookup is just jumping to the address at their ID, counting is just summing, which would probably meet most needs. With this you can handle every person on earth in < 8GB. You can do that with an m3.xlarge on EC2 (15GiB ram) for a measly 25 cents per hour. That's $6/day. That's literally nothing compared to normal server costs.