My uninformed guesses:

1) Hard to price discriminate. Some people are worth almost nothing to FB, others are worth a lot. The price would either have to be absurdly high (e.g. $500+/y) or they might leave a lot on the table from that very profitable minority.

2) PR hit. People more easily accept that a frivolous luxury is only available to the wealthy, whereas even many people who use FB see privacy as more of a basic need. See: response to "price gougers" selling stuff like ice or water at a premium in disaster areas.

3) Hard to cleaning delineate. It's a social graph, your data is useful to generate data on your friends. Could they use it in that case or not? If they do, will that expose them to a lawsuit?

A usual CPC (cost per click) that an advertiser is willing to pay is somewhere between 50 to 150 cents. Let's say it is 100, and you clicked on 10 ads in a month. So, Facebook earned $10 off you. Now, you should be willing to pay more than that for Facebook to prefer the model you are suggesting.

My math above is highly simplistic. For e.g., you'd say you aggressively use an ad blocker and never click on ads. Fair enough, but what about non - tech people unlike us? For e.g., when my dad starting using Facebook at an age of 50+ couple of years, he just tried an unknown plumber through a Facebook ad. Moreover, you'd be willing to pay a monthly subscription, but would all of your friends?

So, what I am getting at is that at Facebook scale they'll earn more though ads than through a subscription model.

This isn't about Facebook selling data. Why does everyone think it is?