Are 5 million Americans irreligious because they use the internet? Allen Downey, a professor of computer science in Massachusetts has been crunching the numbers and reckons that the spread of the internet not only coincides with a great drop in American religiosity but partially caused it. He calculates that internet usage is responsible for there being 5 million more Americans without religious affiliation than there would be otherwise.

This sounds plausible, or at least familiar: it connects with two vague and widespread ideas: that religion is defeated by knowledge, and that the internet is a medium of enlightenment.

There certainly is something to explain. The number of Americans describing themselves as having "no religion" rose from 8% to 18% between 2000 and 2010. How much of this was due to the fact that the number of people using the internet regularly rose from negligible proportions to about 80% of the US population? At the beginning of that period, when internet usage was still fairly restricted, it was possible to believe that there was something intrinsically enlightening about the medium. Nowadays of course we all read the web and know it is full of tripe. If 90% of everything is rubbish, according to Sturgeon's Law, then online, 99.99% of everything is.

The belief that the internet would make us better people was most widespread towards the end of the first dotcom boom: at exactly the time in fact when it was the certain mark of a fool to bet money that the internet would make us smarter. So the rise in American religious disaffiliation corresponded with the discovery that the internet was not a reliable source of enlightenment after all.

Nor has the rise of irreligion coincided with a rise in what you might call rationalism. The self-conscious sceptics, opponents of homeopathy and credulity of all sorts, remain an angry minority. Faith in supernatural beings, however vaguely defined, remains much higher than church attendance. In this country it has hardly fallen at all in the last 20 years.

Downey suggests that the internet might have the effects he wants through two mechanisms: "For people living in homogeneous communities, the internet provides opportunities to find information about people of other religions (and none), and to interact with them personally. Also, for people with religious doubt, the internet provides access to people in similar circumstances all over the world."

Both of these fit squarely into the American secularist vision of atheism as a liberation from the idiocy of rural life. But they also have obvious counter-examples. Without the internet it would be much harder for clan networks to maintain their grip across continents and after immigration. In many cases the internet has made it possible for fanatics to find each other and to co-operate against the rest of us in ways unthinkable before.

There's also the more general point that online life has shown the spontaneous development of almost all the things that make organised religion obnoxious: cults of personality, mindless conformity, furious arguments over wholly unimportant trivia. These are on display on self-consciously atheist sites quite as much as anywhere else, and why shouldn't they be? As soon as you start using theological opinions as a badge of identity, you have stepped into the whirlpool.

But there is one blindingly obvious reason why being online might diminish religious observance and it has nothing to do with ideas. It's simply that every hour you spend online is an hour spent not doing other things. What keeps religious affiliation alive is practice, or ritualised belief. The strongest religions are the least visible ones, because they are so tightly woven into the symbols of every day life. And someone online is almost by definition not performing collective religious acts. Mobile technology might change this, but it hasn't yet. It is the social function of religion that weakens. Belief is an epiphenomenon. What kills American religion isn't argument. It's Facebook.