The neuroscientist Susan Greenfield produced a report last year which suggested that the popularity of online social media was damaging children's brain development, in particular their capacity for empathy.
Shirky has two children, aged nine and six, and says they live in "a very restricted media household", with only supervised access to a communal computer.
"I would not hesitate to say I was addicted to the internet in the first two years. It can be addictive and things not taken in moderation have negative effects.
"But the alarmism around 'Facebook is changing our brains' strikes me as a kind of historical trick. Because we now know from brain science that everything changes our brains.
"Riding a bicycle changes our brains. Watching TV changes our brains. If there's a screen you need to worry about in your household, it's not the one with a mouse attached."
Shirky does not own a television. Americans watch, collectively, two hundred billion hours of television a year, and if online social media diverts even just a fraction of that time, he argues, that has to be a good thing.
"As I say in the book, even the stupidest possible creative act is still a creative act. And I'd still take the most inane collaborative website over someone watching yet another half hour of TV."
By now, despite myself, I'm having to reconsider my old snootiness towards social media.
There's just one last thing, I say. Had I never been online before, and had just read his book, I'd probably be so inspired by his account of the creative and collaborative instincts of the online community, I'd be rushing to log on.
But if I started out on, say, The Guardian's Comment free site, the sheer nastiness of many of the commenters would floor me like a train.
If the web has unlocked all this human potential for generosity and sharing, how come the people using it are so horrible to each other?
Shirky smiles, confident that he has the answer even to this.
"So, there's two things to this paradox. One is that those conversations were always happening. People were saying those nasty things to one another in the pub or whatever. You just couldn't hear them before. So it's a change in our awareness of truth, not a change in the truth.
"Then there's this second effect, that anonymity makes people behave more meanly.
"What I think is going to happen there is we are slowly going to set up islands of civil discourse.
"There's no way to make the internet not anonymous - and if there was, the most enthusiastic consumers of that technology would be Iranian and Chinese and Burmese governments.
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