"Per user" implies a relationship between the mark and the platform - a continuation of the fiction of consent that pervades Facebook and Google. So I'd rephrase your apologia this way:
"FB's revenue per human datum in the US is ~$5/month, but discrete datums who both know they are tracked by Facebook and are rich enough to pay protection money for privacy are few. To protect everyone, they would need to pay $20/month."
"FB's revenue per human datum in the US is ~$5/month, but discrete datums who both know they are tracked by Facebook and are rich enough to pay protection money for privacy are few. To protect everyone, they would need to pay $20/month."