WIREDWherever a net service offers its services in public for free, the spammers gather, writes Danny O'Brien

CRAIGSLIST, THE popular free classifieds site, has a spam problem. A big spam problem. Down in the wilds of the internet, spammers are not only working out how to get around the company's anti-abuse systems, but they're selling their knowledge to other spammers for a buck. As uncovered by John Nagle on news site TechDirt, fraudsters can now buy software that will get around Craigslist's "Please type these letters to prove you're a human" images; software to avoid the company's anti-duplicate post systems; software that will spot when your spammy posting has been flagged as such by real users - and then instantly re-post it again.

Right now, it has reached the point in some cities and subject areas that Craigslist requires you to give a valid phone number - and calls or texts to check whether you're a legitimate user.

Naturally, the spammers are investigating ways to automate that response too.

Wherever a service on the net offers its services in public for free, and for anyone, the spammers quickly gather. Blog comments are weighed down by Viagra salesmen. E-mail sags under thousands of get-rich-quick scams. The solution always looks to be an easy one. Spam comes from being free and open. So you can either close your doors to the general public, or charge a fee.

The fee is the most tempting option. Swapping spammers for an income doesn't seem that bad a deal: it lets you serve the public without leaving an open door to everyone.

But the difference between free and a nominal charge (even just a cent) is enormous - for both sides. For the consumer, there's the hassle and inconvenience of making a payment (on top of whether to pay at all). But that painful burden also sits on those providing the service.

Prior to the net, communications systems such as the phone service frequently spent as much as half of their time and their traffic attempting to maintain the billing systems that authorised the other half. It's pretty much the same with taking cheques on the web. You can open up a free website with a $20-a-month server and an idea. Burying that in even the simplest payment system leads to a far greater headache.

Still, the IT world is nothing if not dedicated to making such systems easier.

There's certainly a common belief that you can eliminate the burden on the consumer side with enough user-interface and billing sleight-of-hand. There are always alternative methods for taking cash. For instance, while trawling through PayPal or a shopping cart online is a pain, paying for ringtones and charge-by-the-minute phone calls are one way of removing much of the friction of payment.

It's not so easy on the retailer side to bolt on a payment system. But then, of course, they are now getting actual money for their work, so perhaps that might offset the pain and hardship of setting up the payments.

But payment transforms a service in far deeper ways than changing its going price. On the consumer side, payment is exclusionary. That really wouldn't cut much ice in a pre-net world ("Oh, am I keeping away people who don't cough up from my product? What a shame!").

But in many net services, the customer also contributes to the overall good of the product.

Think of YouTube: a $1 charge to upload a video or post messages would get rid of the spam, but decimate the number of cool and funny videos that make YouTube a popular destination in its own right.

For Craigslist, it's being free that makes its classified service so inclusive and valuable to users.

And visitors aren't just contributing to the product: in almost all online business models, they are the product. If you have any truck with that other revenue model, advertising, the cost of losing your fickle crowds for what small income you get on the side from billing them just isn't

worth it.

That's especially true when you factor in the biggest externality - the networking effect. Craigslist and YouTube both got to their giant status from a fractional advantage in audience size and reach in their early days; they grew because that audience found it easy to tell others about the great site they'd found.

If you pay a subscription-only online newspaper, how often do you send people links to its cloistered goods? And how angry are your friends when they realise they can't access what you see?

The spam may be killing Craigslist, but being free is what makes Craigslist live. And if they charge, that audience will quickly ebb and flow away to find another free service - one that is probably blissfully (and briefly) spam-free in its hitherto obscure existence.

Spam is the cost of doing business in a free and open environment. And while it's constantly promising to kill and destroy its hosts (be it e-mail, web forum comments or Craigslist), it never quite does. Either an alternative emerges, or the whole system falls into a dynamic equilibrium.

From a moral point of view, it's a pretty horrible "equilibrium" - years of time wasted fighting disreputable, unscrupulous and venal spammers who abuse those providing a valuable service to exploit those too gullible to recognise a fraudulent one.

Inevitably, everyone pays the price for free: but everyone gets the reward, too.