If they can’t manage to charge $1 per user per month for their service then perhaps their service isn’t worth anything at all.
Most companies in this world choose not to willingly leave money on the table, and Facebook is simply taking the same position as millions of other businesses. The only way to get them to earn less than they could is by forcing them to do so through market forces (eg: iOS 14) or regulation.
The mistake here is conflating price with value. The price people are willing to pay is relative to their means. The value, on the other hand, is relative to the utility they derive from it. Moreover, there is an additional external utility accrued to society from having a better educated, healthier population.
That was literally OP's point.