The business model follows from these fundamental differences: a platform provider has no room for ads, because the primary function of a platform is provide a stage for the applications that users actually need to shine. Aggregators, on the other hand, particularly Google and Facebook, deal in information, and ads are simply another type of information. Moreover, because the critical point of differentiation for aggregators is the number of users on their platform, advertising is the only possible business
Source: Tech’s Two Philosophies – Stratechery by Ben Thompson