I don't know how most people find out about YC these days (the channels are certainly different for investors vs developers), but you are probably right that HN is not the first time they hear about YC, though it may be the first real contact with the YC crowd. My hunch is that most YC applicants spend some serious time on HN before applying. It's an effective channel for demonstrating the collective issues of interest to the YC community.

I'm in the 20, then.

Interesting...in my techphilia, I found HackerNews through just following interesting stories, and landing on a singular discussion, which is now a daily obsession of going through the posted articles :D

This exposure is one of those hard to ROI measure functions (press) etc.