I also love Pinboard, as well as joshu's original del.icio.us, and all services that find that completely narrow utility that I like to call "stripped and liberated". Stripped in the sense that they seek to do one thing well, on the face of humanity, forever. Liberated in the sense that they are liberated from the need to be beholden to investors or broader corporate strategies which shred their utility and focus in service of some other imperative.
Has the day of the "stripped and liberated" service come and gone? Or are there still opportunities waiting to be discovered?
Also, I love Pinboard so much that I wrote an Android app (https://play.google.com/store/apps/details?id=in.shreesh.pin) which I plan to improve/fix.