You could think of Oculus as an effort to increase the amount of user attention inventory Facebook has in supply to rent out to advertisers. Also, they are positioning themselves as an intermediary for transactions in the future VR economy by being platform owner (a la Apple owning the App Store). My guess is the long-term plan is to make money here by displaying ads in VR and by getting a cut of goods and services sold though VR (see: Ready Player One).