The word "luck" suggests a chaotic nature to the outcomes; the initial conditions being the time of day of the post, the first few up/down votes, what people ar

The word "luck" suggests a chaotic nature to the outcomes; the initial conditions being the time of day of the post, the first few up/down votes, what people are online and browsing at the moment. Of course controlling for quality content, as you say. But there might also be some intrinsic aspect to successful posts that's hard to analyze and replicate and that is entirely orthogonal to quality, such that that post would have made it to the popular page even under different initial conditions. So it's "luck" in the sense that it hits some sweet spot in the fractal plane of our imagination.