While playing Multiplayer, the mobs tend to not spawn when split up between other players. If a player has most of the mobs spawned in their area, and another player goes to explore, mobs tend to not spawn. I don't have direct proof of this happening, but I did tend to notice a lack of mob spawning while I was exploring. At night, no monsters were spawning, and all the mobs that did spawn were near the area where we built our house.

Thank you,
Jones989