Back in the early days, the question was
How is wikipedia new? How does its amazing social structure work? How does the Biggest Document Ever get written without an editorial team or clear hierarchy?
But now that Wikipedia's the top google result for half the words in the english language, the question is
How does having Wikipedia at your fingertips change you? How does it change society?
* * *
Huckleberry Finn has been banned various times for various reasons — pretty much continually from when it was published. But at first, it wasn't banned for use of the N-word, or for undermining racist southern norms. It was banned because it used the word ain't — in other words, what shocked people was deviation from linguistic norms.
From 2007 this looks silly, but in the big picture, being shocked at the use of "improper" English serves an important social purpose: it slows the evolution of the English language.* If this seems unimportant, remember that without intervention, languages evolve at a ridiculous, breakneck pace.
Consider hunter-gatherer tribes in the tropics, where they can be totally self-sufficient: if half a tribe splits off, and sets up camp a few miles down the river, its language will be unintelligible to the original tribe within two generations.
If English evolved that fast, we wouldn't be able to read Twain, let alone Shakespeare. We wouldn't be able to pass down any knowledge at all. Everyone understands, on an instinctual level, how much that would suck — hence the constant debates about the "real" definition of words among everyone except people who've taken linguistics classes.
* * *
Maybe one social purpose of wikipedia is to provide canonical definitions of "unofficial" words and phrases that would otherwise be left to the drifting winds of spoken transmission.
For example: everyone who grew up in the '90s thinks Atlanta's ban on sagging is really strange. It doesn't make sense as enforcement of obscenity laws because Atlanta didn't care in 1998, when everyone and their mother sagged. And it doesn't make sense as evil racial profiling because Atlanta didn't care in 1994, when only black people sagged.
So what changed? Maybe the difference is that politicians can type "sagging" into google and read a wikipedia entry that says "Sagging has its origins in the prohibition of belts among incarcerated inmates, which was emulated as a fashion statement by non-inmates for its tough guy cachet."
Nobody in the '90s knew that was the origin of sagging. They just knew it made you look cool. The definition had drifted far from its origins, but Wikipedia ties it back. 15 years after sagging escaped from prisons, Wikipedia has dredged up the the collective unconscious and made it solid.
* Or maybe not. See the comments -- any linguistics professors in the crowd?