The people willing to pay for this have more expendable income, and are more likely to take out their CCs and pay for something online. Ad-driven services like fb/insta will make more money from this demographic, and so their revenue "per user willing to pay for no ads" will be significantly higher.

Yes. Ironically, paying not to see ads only drives up the value of our attention even higher and makes them want to advertise to us even more. Paying to avoid ads is really just paying for the privilege of doing free market segmentation on their behalf.

Per quarter, not per year. That is also the average across all users, while only the power users (ones that make up a majority of the ad revenue) will pay for a subscription, so their ad value is a lot more than average.

If the average revenue per user is then 72$ per year, then, it follows that the appropriate price is 6$ a month, not 12$?

Read the second part of what I wrote. If every existing user was forced to subscribe then $6 would be appropriate. If only people who earn them $50/mo in ads end up subscribing (which is a lot more likely) then Facebook will lose money on this option.