I'm not sure we have any data along those lines. ComScore may have something perhaps. 

To expand on the qualitative side though...

- As you say, Facecbook and Wikipedia are very different things. Social networks are inherently fickle pursuits for young people. It's easy to forget that Wikipedia is actually older than Friendster, MySpace, and Facebook. Social destinations inevitably wax and wane because they're based on what's cool, while Wikipedia is not cool. Wikipedia is a utility. That's true for people of all ages.  

- We are pretty certain that, when it comes to high school age kids and college students, Wikipedia's utility has not been replaced by some other reference work. 

- We know for a fact that very young people have always been part of our editor communities. We've had and continue to have even teenage admins, bureaucrats, and others in leadership positions. That's pretty remarkable. 

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