Do you like David Cameron? Do you "like" David Cameron? Not in a high-school way: I mean on Facebook.

Just a month ago, the prime minister's official Facebook page had barely 60,000 likes; it has now got almost 130,000. That's 50,000 more than Nick Clegg, and 100,000 more than Ed Miliband.

All that raises questions. First and foremost: why do any of them have Facebook pages? Don't they have countries and/or parties to run? But also: how did Cameron pick up so many likes, so quickly?

It seems the answer is adverts. Conservative party strategists paid Facebook to advertise the PM's page on the site; the Mail on Sunday estimates the cost was about £7,500.

You can choose to pay for ads on Facebook in a number of ways. Billing per click and per thousand impressions are both popular, as they are on the web in general, but you can also charge per "interaction". That can be people clicking "attend" on an event, or liking a page. At about 50p per like, this doesn't come cheap. But the brutal reality of Facebook advertising is that it works.

For reasons which are too arcane to get into here, Facebook thinks I run a business. As a result, I occasionally get given free money for adverts, to tempt me into using the site to promote my non-existent business. Typically, that money instead gets pumped into low-effort pranks. To date, I have linked several hundred people to a shock website; shown around 250,000 people a poor pun about George Osborne; and, in a particular favourite, targeted an advert exclusively at Conservatives urging them to read the Communist Manifesto. Seventeen clicked through.

Facebook gives extremely detailed feedback about how people have interacted with such campaigns. And what you learn, very quickly, is that people do interact with Facebook adverts, in huge numbers. Even if they're obviously jokes.

At least in buying likes through an advert, Cameron ensured that he's probably getting them from actual British voters. If you're prepared to go to the shadier corners of the internet, £7,500 will buy you around a million Facebook likes, but – for once – Cameron appears to have gone for substance over style.

A million likes would give him a healthy lead over Clegg; but they would come from "click farms", where low-paid workers in developing nations are given as little as $120 a year to click "like" on page after page after page.

Or he could have just skipped the middleman, and gone to a site such as Amazon's Mechanical Turk to offer a penny for every like. The people who trawl Mechanical Turk looking for paying jobs are frequently just as low-paid as the click-farmers, but the shiny web interface offers the illusion of a future free of exploitation.

Of course, Cameron may be perfectly happy with buying a million fans from Bangladesh. It would still be quite unlikely to translate into success at the polls.