His website pricing gives a hint as to how much the app has made him:

> The [Pinboard] fee is based on the formula (number of users * $0.001), so the earlier you join, the less you pay.

The current price is $9.22, so following this formula he has ~9220 users, which has paid him roughly 42K. He also charges $25 (minus your sign up fee) for archiving, which would bump that number up a bit.