Facebook is a massive traffic monster. As of this writing, it has over 1.3 billion, and that's with a capital B, daily active users. Think about that, over 1.3 billion people using the same website day after day. On top of this, it continues to grow.
Also, when people are on Facebook, they pretty much stay there, and they view page after page of content. In fact, on a month to month basis, as of 2017, Facebook has slightly over 2 billion monthly active users.