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> you make a profit of P per user

I believe the point of the quote is that they're making a loss per user

These are not losses. Losses are, as in Twitter's case, when P is negative.

GP is surmising the case where marginal profit per user is positive, even though average profit per user is still negative.

IOW, calling it I, net income per user, would have made the argument more clear.

By golly you are right! They just need a complex count of users and negative complex profit and the result is positive!