Clay Shirky is a genius at the top of my list of people I would love to meet, others close are Cory Doctorow and Yochai Benkler.
The video embedded in this page (link to video on blip.tv) is from a speech he gave at the Web 2.0 conference 2008.
He released a lightly edited transcription of the speech on the blog of his new book, under Creative Commons By-Attribution ShareAlike licence. Since my blog is under the same license, I’m going to legally copy and paste some parts of it here but I suggest you to read it entirely and to watch the video.

The critical technology, for the early phase of the industrial revolution, was gin (…) the transformation from rural to urban life was so sudden, and so wrenching, that the only thing society could do to manage was to drink itself into a stupor for a generation.

If I had to pick the critical technology for the 20th century, the bit of social lubricant without which the wheels would’ve come off the whole enterprise, I’d say it was the sitcom. (…)
And it’s only now, as we’re waking up from that collective bender, that we’re starting to see the cognitive surplus as an asset rather than as a crisis. We’re seeing things being designed to take advantage of that surplus, to deploy it in ways more engaging than just having a TV in everybody’s basement.

So how big is that surplus? So if you take Wikipedia as a kind of unit, all of Wikipedia, the whole project–every page, every edit, every talk page, every line of code, in every language that Wikipedia exists in–that represents something like the cumulation of 100 million hours of human thought. I worked this out with Martin Wattenberg at IBM; it’s a back-of-the-envelope calculation, but it’s the right order of magnitude, about 100 million hours of thought.

And television watching? Two hundred billion hours, in the U.S. alone, every year. Put another way, now that we have a unit, that’s 2,000 Wikipedia projects a year spent watching television. Or put still another way, in the U.S., we spend 100 million hours every weekend, just watching the ads. This is a pretty big surplus. People asking, “Where do they find the time?” when they’re looking at things like Wikipedia don’t understand how tiny that entire project is, as a carve-out of this asset that’s finally being dragged into what Tim calls an architecture of participation.

(…) a project started by a professor in Brazil, in Fortaleza, named Vasco Furtado. It’s a Wiki Map for crime in Brazil. If there’s an assault, if there’s a burglary, if there’s a mugging, a robbery, a rape, a murder, you can go and put a push-pin on a Google Map, and you can characterize the assault, and you start to see a map of where these crimes are occurring.(…)
Maybe this will succeed or maybe it will fail. The normal case of social software is still failure; most of these experiments don’t pan out. But the ones that do are quite incredible, and I hope that this one succeeds, obviously. But even if it doesn’t, it’s illustrated the point already, which is that someone working alone, with really cheap tools, has a reasonable hope of carving out enough of the cognitive surplus, enough of the desire to participate, enough of the collective goodwill of the citizens, to create a resource you couldn’t have imagined existing even five years ago.

So that’s the answer to the question, “Where do they find the time?” Or, rather, that’s the numerical answer. But beneath that question was another thought, this one not a question but an observation. In this same conversation with the TV producer I was talking about World of Warcraft guilds, and as I was talking, I could sort of see what she was thinking: “Losers. Grown men sitting in their basement pretending to be elves.”
(…)At least they’re doing something.

Did you ever see that episode of Gilligan’s Island where they almost get off the island and then Gilligan messes up and then they don’t? I saw that one. I saw that one a lot when I was growing up. And every half-hour that I watched that was a half an hour I wasn’t posting at my blog or editing Wikipedia or contributing to a mailing list. Now I had an ironclad excuse for not doing those things, which is none of those things existed then. I was forced into the channel of media the way it was because it was the only option. Now it’s not, and that’s the big surprise. However lousy it is to sit in your basement and pretend to be an elf, I can tell you from personal experience it’s worse to sit in your basement and try to figure if Ginger or Mary Ann is cuter.

Let’s say that everything stays 99 percent the same, that people watch 99 percent as much television as they used to, but 1 percent of that is carved out for producing and for sharing. The Internet-connected population watches roughly a trillion hours of TV a year. That’s about five times the size of the annual U.S. consumption. One per cent of that is 100 Wikipedia projects per year worth of participation.
I think that’s going to be a big deal. Don’t you?

Read the rest on Clay’s blog so you get to know the story about the mouse. I think it is worth your time.