I was in the middle of a harsh bout of unemployment after the startup that hired me out of school folded unexpectedly (literally in a week's time). I was going from contracting gig to contracting gig but got tired of chasing people for money.
On a whim, I responded to just one such thread on HN. I got no recruiter spam, but rather an email from a company (I won't say which for privacy reasons) that totally changed my life. Great recruiting process, interesting and challenging interview gamut, and they made me an excellent offer contingent upon relocation, which I was hoping to do anyway.
So, the answer to the question is YES those threads work IF you work them. Stay on top of your emails, follow up, be diligent, study hard. It's so so worth it.