The closest thing I can see to direct monetization of HN is that YC uses it as a captive advertising platform.

They don't serve 3rd party ads, but they use it as a job board (with closed comments on those posts, LOL) for their companies, and to announce product launches and such. All that stuff gets boosted, nobody's organically up-voting a comments-disabled job post from some mediocre startup.

[EDIT] The less-direct part, yes, is stuff like brand awareness and community goodwill.