Craigslist has long had problems with spam. The one feature that has made the site a success, the ease at which one can post, has become a hindrance as spammers and law breakers have taken over. In early March Craigslist began what is now turning out to be an incredibly effective anti-spam campaign by using phone verification on posts. In an initial effort to rid their “erotic services” section of prostitution and pornography ads, Craigslist has now slowly expanded the system to other service categories within the site.

The system is simple and, thus far, largely unavoidable. When a user attempts to make a post, Craigslist asks for verification with a viable phone number. It then automatically calls the number and issues a verification code which must be re-entered for the post to go live. If the post gets flagged, the phone number is blacklisted, preventing any further posts from that number. The key to this is that, while email addresses are cheap and disposable, phone numbers are not.

This could be a nightmare for Craiglist advertisers as the service expands, and they have yet to find a way to circumvent it. One popular idea has been cheap VoIP numbers, but VoIP services usually charge a fee of a couple dollars after the third or fourth number. This quickly adds up when posting hundreds of advertisements. The only affordable solution thus far has been using public pay phones.

The issue has been around forever on the web, and most solutions involve either moderation within the company, or user and community moderation (which is the model Craigslist adopted). There is a careful balance in trying to keep content clean, but at the same time not taking away the main attraction of the site. Using a phone number is just a short-cut way to establishing some form of identifier token, rather than using email addresses.

Though this should drastically limit spamming for Craiglist, the service may make honest posters think twice if they must publicly display their phone number. It also could prove to be a nuisance to honest users who are illegitimately flagged. In the end, the service gives direct results, and should keep Craigslist largely spam free for now, although at a slight cost to user convenience.