HN is many things to many people, and to some it is definitely a social bookmarking site. It even has something called a 'bookmarklet', if you have a description of HN that does not include the social bookmarking aspect (posting links and discussing them) then I'm very interested in to what you think HN is.
When people get into discussions about the meaning of social bookmarking, I often giggle remembering an early del user who wrote in to complain about the inappropriateness of some of the links because he thought 'social bookmarking' meant 'bookmarks on social issues'.
I'm not even sure how to get stuff on or off that list.
Again, in what way can you use this site as your bookmarks? You save an obscure documentation page and a mod deletes it, and then what?
HN (like digg, reddit, etc) is social news, or something. People who confuse the two think so because the delicious front page and digg looked similar, or whatever.
Obscure documentation pages are most likely not going to survive, I agree with you there. But I definitely do use it to 'bookmark' technology news and it is very rare that a link gets deleted.
The bigger risk for deletions seems to be not tech related stuff but rather non-tech related stuff that manages to get a couple of upvotes.
HN is social news simply because the audience can define it as such.
The main ingredients, the ability to post links and to discuss them and to revisit them at a later stage makes it such that some users will come by and use it in that way.
All the stuff you submit and upvote goes on that list, you can't remove stuff from the list if you've upvoted it, you can't remove stuff from the list after a certain time has passed.