I could go a couple different directions from this thought.

1. Science can't prove to you that anything exists other than your own observation. Nobody can even prove to you that your own body actually exists. All the other people? Well, dreams have people too, don't they?

2. Assuming that any of the other people are real, the population doubling rate would require double processing power to run the NPC characters, if some of the other people are NPCs. This could be mitigated by requiring each user to process an amount of NPC's on their own hardware, like a decentralized VR game.

3. Drawing upon a short story called, "The Egg", YOU could be all of those people that you interact with, including the NPCs.