I did a back of the paper calculation as to how much fb would earn via me as a user. I noticed a user like me spends about 15 mins a day on facebook and see around 6 sponsored posts. Internally FB ads are all CPM based, at about $4-5 per CPM, no matter the optimization advertisers choose. In a year he would have seen about 2000 impressions, and hence minimum price would be around $10 per year. (There are many users who spends a lot more time and hence the revenue multiple will be higher)

But there is more revenue than just that. FB allows advertisers to reach specific audiences, and even create lookalikes, so overall, there will be a %age of the users seeing more ads than me, potentially personalized for them, and average revenue per user would be higher.