Do not limit yourself to just this recruiting strategy. We only post on the free sites. My dad always told me you get what you pay for. Headhunter.net, the largest free site, just changed their business model to a fee based site. Companies that are fee based have money to reinvest in their technology.
Many sites start off free but they begin charging once they start getting some traffic.
I am not downplaying the importance of using free sites. It is a big mistake to only use free sites. Fee based sites are very reasonable priced in comparison to other traditional recruiting methods. In most major cities you can barely run one Sunday newspaper ad for that. If you get a few hires from a fee based site, you will reduce your cost per hire significantly.