Sometimes I check out the Mixergy podcasts out out by Andrew Warner. Its a podcast series about entrepreneurs/startups. Recently he had on the founder of Wall Street Oasis, an online community for wall street types.
In the interview he reveals that they only get about 10% of their revenue from job postings. Just 10%!
The other revenue comes from advertising, affiliate programs, etc. So this is a good lesson about starting a community site rather than just a job board. You need to build a community which will allow you to add things such as job postings into the mix.