They have quite a lot of monthly visitors there. They have for some time now. Maybe they go out and find advertisers, and charge them montly rates??? You could pretend you are an advertiser and send them an e-mail asking about their advertising package.
There probably trying to get a user base, than cash in once they have tons of traffic. Look at craigslist. It was free for a long time. They just recently started charging for job listign ads and are raking in the money now. Myspace and youtube were the same way.