I'd expand that beyond ads alone, to have the payment prevent tracking, profiling, "filter bubbling", and sometimes emotional manipulation experiments. [1] But that model won't scale, and it wouldn't offer Facebook growing value over time either.
If you put a monthly fee for ad free service, probably your mostly valuable customers (those error m with money to spend) are first to switch.
Facebook can probably generate vast amounts of profits from the users if it manages to be part of the actual transaction, not just an ad platform. The same kind offer thing Google was planning with Google Checkout. IIRC the idea was they would not charge for clicks, but would get payment of customer actually makes the purchase.
Think about Facebook first creating for you the urge to buy something (by manipulating you feed) and then selling the thing or taking their cut off the transaction.
You could still have a free service that displays ads.
Apple's track record of not needing to track me or sell my personal information to prop up its business models, because they launch products that have real "get the end users to pay money for it" business models, is one reason why I stick with their products.