I automatically assume that astro-turfing happens on a large scale on every public forum. HN has been around long enough and has enough high profile people on it, to make it a target. The moderation is generally quite good, but unfortunately as the astro-turfing ramps up, the moderation will need to make HN a blander and blander place as any form of opinion is censored. We aren't there yet though, so enjoy it while you can!