[seiji]

You can do clever things with word aligned hybrid bitmaps to heavily compress sparse bitmaps.

Though, if you are storing binary users, 700 million users take less than 90 MB of space (assuming a straight array implementation). Old days wouldn't need to be kept in memory. If you want to think beyond redis, you could keep the active data in redis then flush old data to disk for later querying.

Sounds useful if your users map to a (0, Max] integer representation. Sounds complicated if you use uuids or external vendor IDs for users anywhere (you'd need an intermediate mapping table somewhere).