But to exclude popularity from the definition of success is highly atypical, at best.

"My son is a successful artist, and like Van Gogh he's only ever sold one piece of art in his life" said no mother ever.

Now post mortem, Van Gogh has certainly "enjoyed" more success, by many peoples' rankings.

It is certainly true that one of the ways in which success can be measured is sales.

What is Y Combinator's HN selling? Are they successful in that effort?

HN isn't selling anything, but without HN the founder feed would likely dry up (or maybe not, now they have critical mass) and eventually of course they're selling stock.