I've never been hired from IRC though.
HN who's hiring, like most online job advertising, is a crapshoot. It's a once-a-month post where you are drowning in noise. Finding a very large group of potentially relevant people for the roles you are hiring for isn't as straightforward as it sounds. Engaging said group and convincing folk from said group to come work for you is exponentially more difficult.
>>When was the last time you engaged with a company with a view to working with them because you saw an ad they happened to post in the right fora?
So, yes, hn is where I engage with a company with a view with working with them where they happen to post in the right forum. Suddenly your point is that it isn't a good place to find jobs? Maybe not, but that wasn't your original point. You're moving the goalposts.
Places like HN and relevant IRC channels are great if you're hiring for one or two roles but beyond that then it's a bit of a lottery.