database sharding for user profiles, how to split

Our user profile database is getting way too big, and we're struggling to scale it. We're going to shard it, but picking the right way to do it is tough. Should we split it by user ID? That's easy, but we could end up with some shards being overloaded if a few users are super active. Or, should we go by geographic region? That would spread things out better, but it makes querying across shards really complicated. Has anyone else dealt with this for big systems? I'd love to hear your stories or any advice on how to make this decision.