1. I'm a big fan of the wiki community. Aside from Wikipedia I regularly access others as well. A few of my favorites are http://starwars.wikia.com/wiki/Main_Page or http://fallout.wikia.com/wiki/Fallout_Wiki I use these either as simply a fan, or for finding that last hard to unlock achievement. They are great sources of information as initial places to go look. I don't take their word as gospel, but they provide a great jumping off point to any research process, even academic research. Last term I was doing a paper on the video game Dragon Age: Origins, and found some great sources in the reference links from the games Wikipedia page.

2. I don't think that Wikipedia can conquer expertise. The thing is that much of what draws users to Wikipedia is based on the work of experts. Most articles on the site have references to academic or first hand sources and those that don't are labeled as needing citation. In this way the mob will never conquer expertise, it simply finds new ways to reinterpret, distribute, and make sense of what the experts produce.

3. The hobby impulse is my first guess. Its not that people are doing it for others, its that they get a level of personal satisfaction out of the act that is a reward in its self. Given that I don't participate in this on a digital level I can't comment extensively. I do it on an interpersonal level with occasional volunteer teaching of martial arts classes, but mostly that's just me looking for a group of people to work out with and they happen to be interested in joining me.