My main point was that a 5$ ONE TIME charge is way too low. Typically ARPU is calculated over a certain period of time (monthly, quarterly, yearly).
That must be the profit margin. $5 isn't enough to cover costs and living I think.
$5 per user is equal to $5,000,000,000 for revenue and I think Facebook made $1,000,000,000 net profit last year, so Facebook makes about $4.5 per person gross and about $1 per person profit at the moment.