It is agonizing just how much data vanishes each year due to abandoned hosts and domains. The saying that "Once on the Internet, always on the Internet" is hogwash. I'd estimate that at least half of all my bookmarks that are 5+ years old are now defunct and the ones that I bookmarked a decade or more ago are almost entirely gone.
Archive.org is lousy for retrieving most database-driven and dynamic content as well as files themselves. These forums alone are at least the third incarnation of such (darkbasic.co.uk and RealGameTools were the predecessors and very rich in years of content that is now all lost and forgotten). I have been a member to many forums that simply up and vanished without notice and never mirrored or cloned, some with hundreds of thousands of posts that were not archived anywhere.
I often find myself going on long nostalgic binges of trying to dig up old sites I have fond memories of and have a limited success rate. Archive.org itself didn't start archiving sites until years after WWW was popularized, so there is an even larger sector of completely unindexed content lost forever.
