The answer is, of course, Facebook, which was launched in February 2004. But no one knew that until a couple years later, when it opened its doors to the general populace.
The true competitor to Wikipedia will probably look nothing like Wikipedia. It will likely appear "suddenly" to those not in the know. And Wikipedia probably won't die die... After all, Myspace still exists. It will just be a place no one goes to anymore, or maybe as a last resort when other options are fruitless.
