Maciej Cegłowski, the owner/operator of Pinboard, shares about the various business models of bookmarking sites, and why he chose the model he did for Pinboard. The PDF is based on a talk he gave in February.
The heart of this PDF can be summed up on Pinboard’s about page:
It boils down to this: running a bookmarking site costs money. If you’re not paying for it, then someone else is, and their interests may not be aligned with yours.