Why did Mark Zuckerberg join forces with Sean Parker and left Eduardo in the dark?

It was really late and I was sleepy and they were talking WAY too fast. Eduardo was providing the funding or something but Mark felt he was doing an insufficient job and felt this would hinder the momentum of the facebook project. I think Mark wanted to make the facebook a much wider initiative so is that why he employed Sean Parker to get the job done? And the irony I guess is that, Mark trying to build this revolutionary friend-making utility, ultimately cost him his only friend. Or something like that?