The HN crowd self selects to people deeply interested in tech. For instance, there's many VC's and founders lurking or public in the comments section. Never seen that anywhere else.
I wouldn't think about it much. The population on here is more likely to be VC/founder/VP than other internet forums but 95% of those people still aren't on HN. Too small sample population
Probably pretty common, I did/do. I know several other C-Level/Directors that do, but probably only in the tech/internet space.
It's probably about as common as being a CTO/VP/Director level employee within the general population. Most HN users are likely individual contributors; this is because most people are individual contributors.
(I did know one VP at Google who read HN. Only one, though.)