That's true actually. pg had been interested in social news sites for years (Slashdot because people would post his essays there, but especially Delicious). He gave the Reddit founders the idea for Reddit, and for a while was their number one user. But once Reddit started taking off it turned in a direction that he wasn't as interested in and they stopped implementing his feature requests, so he built his own. That was one of the motivations for HN. The others, that I'm aware of, were helping to grow the nascent YC community, and writing a significant application in Arc.