I feel there lots more than what we see on HN. Countless calculator, weather and niche sites I come across who never have anything in public. Long back each niche would host a forum software and would make quite a bit through adsense. Ofcourse, Facebook etc al have pretty much killed that avenue.

I do think there's a lot more than we see here, or that any one person can see anywhere. I also agree that ad revenue doesn't really work without substantial traffic. Really, Reddit has usurped many traditional forums.

I tried to monetize an Android app with ads and a paid version. I could never gain enough users. I had a second app with more users, but didn't want to clutter it with ads. Of course that one actually had a decent number of users, but infrequent use (designed for utility, not monetization).