I think that in order to create a mobile site, one must first research the audience and see what kind of sites they visit on their phones. While in Japan, I only visited practical sites like those that located local businesses, gave me train schedules, or the weather forecast. I think that social sites are also popular, but that would be a difficult market to break into.
A job site seems a little too serious to browse on a mobile phone, but there's probably a niche market for it. Jokes, on the other hand, are a bit frivolous, but again there's probably a group of users that would visit such a site. Basically, you're looking at offering content vs. service.