The guy that runs it doesn't seem to be in it for the money. It looks more like he's trying to run a no-frills bookmarking service with a sustainable business model. He regularly, openly, and humorously criticizes other sites that get bought or go "tits up" and blow away all user data (via the @pinboard twitter). The service used to have a one time fee, and the one-time fee would increase slightly for each user. He has since switched to $11 yearly because users were getting too confused about the one-time fee increasing.