Based on their signup help page (https://pinboard.in/help/fee/) the price is "number of users * $0.001". Now there is no telling when they started with that pricing but assuming it was from the beginning they have made around $50k. Since it was created 3 years ago that is an average of around $17k/year.

I realize the archival accounts are $25/year and I have no way of knowing how many they have but assuming it doubles their profits which I doubt they are still making less than $40k/year minus expenses.

I'm not huge on selling out but I would rather build a large app and flip it vs run a company that profits less than $40k/yr

(I did not double check my math so please forgive me if I rushed through this too fast)