Paid advertising for a free site?

Greetings, My web site has been building users on "word of mouth" advertising and links in "free directories" since it's launch in June of 2004. We have been doing pretty well on our own. My site is a free gay personals and chat site with no membership costs. But with about 25,000 users we are still small compared to sites like ManHunt, OutPersonals, and the grand daddy of them all, Gay.com. Once we get users in we have no problems keeping them since we offer everything those bigger sites do but completely free.

So we are looking for new ways to promote our service. I have searched around and looked at a lot of "paid advertising" options. Most of these are priced well for sites that stand to gain membership fees from new visitors. But it just don't make sense for a free membership site to shell out these large amounts of money for such a small return.

So what "paid advertising options" give the greatest bang for my buck? Where can I go to spend very little and get massive traffic in return?

Also what is the best kind of traffic programs? Pop-ups and Pop-unders seem as if they would be a complete waste of money. I mean who doesn't have some kind of pop-up blocker by now? I was thinking pay-per-click or banner would be the best. But they always seem to cost the most. Is there really such thing as "silver bullet advertising"? Or is it just silly for a free site to pay for any kind of traffic program?

I agree that paid advertising for free sites is a difficult proposition, return-on-investment-wise. But if you have a "sticky" free site where you get loyal return visitors, it may be worthwhile. Your personals sites are sticky, so if your motive is to grow and you can afford it, buy some high-exposure advertising; don't expect immediate return. (I don't know which paid advertising would be best for you.)

Have you used all free-advertising options available? I know you do link exchanges and link lists; but how about banner exchanges, small-free sites, AVS sites, and TGP gallery pages (these can bring in lots of traffic) all to promote outboy?