I know what you're saying, that actual retail sales are not a big deal for them. However, you're making a category error in that "Windows purchases" (as well as Office, etc.) are what defines their customers of Microsoft's software arm where there is no parallel situation on the Facebook side. That is, at all levels, Microsoft's customers are interacting with the same stuff: Microsoft software.

For Facebook it's different. Facebook's customers are consuming and interacting with an entirely different resource than Facebook's users. In fact, I'd say that Facebook's customers (advertisers et al) actually have very little social interaction with each other on an experiential basis compared to Facebook's users. This is to say that Facebook likely puts a lot of work into ensuring that nothing on the customer side gets inadvertently shared, unlike the user side.