The essential difference between social networks and forums - and Google or Facebook could do this in a heartbeat, and probably will at some point, which is when places like this will go away forever - is archive-ability. You can't stumble across a Facebook conversation from a week ago and contribute to it, let along a month or a year ago.
This an interesting point, but as I was recently at a conference listening to Cory Doctrow, Jimmy Wales and a number of others, and they illustrated that this sort of thing cuts both ways in terms of appeal.
Not everyone wants everything they've said archived in a very accessible way, and publicly analyzed to pieces. With Twitter, if you say a dumb thing, whilst it may still be accessible, it will be reasonably hard to find, and people simply can't analyze it to pieces due to the 140 character length, and so you will just have simple, largely contemporaneous responses.
That's not a bad thing for someone like a game designer, particularly, who doesn't necessarily want to deal with "OMG BUT WHAT DOES THIS MEAN!?" (followed by hundreds or thousands of words of guessing!) being attached to his every announcement.
So I'm not sure that future social networks and so on are going to really want to increase the ease of access to archived/non-contemporary material. On the contrary, I think we may well see more putting it into some kind of "deep storage" (which it is invisible to search engines), or even deleting it outright. Annoying as that is for studying the history of something, I think it better reflects how humans want to operate (this is particularly shown by young people today, who eschew services with strong archiving and so on, and prefer more ephemeral ones).
Which is not to say ENWorld should change. Forums are a cool thing and serve a purposes, but I think there's value in weaker archiving (or deeper/hard-to-access archiving) and even deletion of older stuff.
I think a lot of forum posters would actually be a lot more reasonable, too, and less dogmatic, if their posts of a year, or five or ten or twenty years ago could not be dug up in seconds. We seem to sneer at people who change their minds, learn from arguments and so on, and the nigh-permanence of forums tends to reinforce this tendency, I feel.