Assuming that the only users who would be willing to pay are those who use one of FB's services at least once per month; then FB would need to charge $26/yr to

Assuming that the only users who would be willing to pay are those who use one of FB's services at least once per month; then FB would need to charge $26/yr to match their current revenue. Given that they already have 2.7 billion MAUs it's unlikely that they could grow their way out of such a change, and serving ads is ultimately inexpensive.