At Internet scale (~2 billion connected users) it's minuscule.

0.005% of all available users.

Within the United States, it would fall well below the 100th most widely circulated print magazine, as a monthly circulation statistic. I'll assume that monthly stats are a few times higher than the daily usage metric, but you'd still have to hit roughly 1 million to break into the top 100. https://en.wikipedia.org/wiki/List_of_magazines_by_circulati...

From an advertising perspective, unless you're extremely focused in your interest base (and there are markets of interest within the HN readership), it's tiny.