Facebook's biggest asset is their data. My guess is that if they can't show you ads, they lose some of the ability to track you and your behavior in terms of interaction with the ads, interaction which creates additional valuable data. Due to network effects / Metcalfe's Law, if you eliminate ads for X percent of the user base, this will have a disproportionately larger (downward) effect on the value of their data in aggregate. Which may not be totally offset by whatever revenue you can collect from users directly in premium subscriptions. I would bet they have surely done the math on this.