The much awaited "monster-hunting" game will be a Single Player game. However, despite the game being a single player, Horizon: Zero Dawn has some "social features" making players of Horizon: Zero Dawn feel connected to each other.
What are expected?
What these social features are remains a mystery but we can expect Leader boards, an online trading system for the weapons and gear you collect from monsters and an online co-op mode as one of those social elements.