That is, if their business model depends entirely on ads. I think that the net gain of allowing services to complement their web app will more than make up for

That is, if their business model depends entirely on ads. I think that the net gain of allowing services to complement their web app will more than make up for the loss of a small portion of ad views by making their application as a whole more compelling. By deliberately and aggressively excluding themselves from these services, Facebook only stands to lose users.