The Archive Team[1] tries to scrape such records when the sites are in danger of closing down. Currently my Warrior[2] is pulling pictures from Panoramio[3]. Unfortunately, resources are often scarce, particularly storage.
I believe they're currently preparing on working on Vine, and, from what they wrote on Twitter just today (can't search for it right now, don't have a stable connection right now), they don't have any problems with the storage at the moment thanks to Archive.org.
I suppose we could email them with a request to make sure they scrape everything off of first, few pages of HN or key subreddits. I'm not sure about Stack Overflow where they or Google would know best based on hits certain questions get which are best to archive. Maybe just archive whole thing. Sites might even pay them to as a high-availability or backup option. Low odds there but asking doesn't hurt. ;)
I've seen lots of omissions in archive.org. Not that it's not great. I love it. But there's a lot more that needs to be archived than what's in there.