Maciej publishes a yearly progress statement [0] with revenues and costs included.

[0]https://blog.pinboard.in/2016/07/pinboard_turns_seven/

The website claims to have 25k users [1].

[1]: https://pinboard.in/tour/

I'm happy to be one of them. I honestly barely use it, but I had a ton of bookmarks at Delicious that I was uncomfortable about losing. $11/yr means they stick around, plus I do still stuff links in there every now and then.

The true number of accounts is probably higher since anyone who bought in before he switched to annual billing paid a one time charge.