Hi, my name is Felix and I'm addicted to wikis. Or used to be. At some point I was running five of them at the same time, based on three different engines, one of them my own experimental creation. Time has passed, things have changed, and I'm down to just the one you're reading right now, running PmWiki.
(Ironically, not long after writing the above, I started another, based on OddMuse. Oh well.)
More importantly though, wikis are part of New Media, and arguably the first form of social networking native to the web (which message boards predate by more than a decade). Nowadays a dying medium due to spam and the general difficulty of keeping one active, wikis nevertheless still power some of the largest online communities, such as Wikia, and many GitHub projects use them extensively.