Marketing ideas for a free website in todays money hungry marketing companies.

I have been on the net for a while now I am in the Social/Dating site game. I have had my site for about 8 years and the past 5 years ive seen a drastic decline in what you get for your money these days for marketing. I have been able to generate myself alot of users with very little income in marketing in the past as in back in 2003. Now a days I have spent literally thousands on advertising with very little user signup. Where as back in 2003 I could get 1000 sign ups for 140 bucks. I believe this is because we are now all subject to CPC prices which are super inflated with little results. So heres my question for you all. My site is free so what would be the best thing for me to do to market. I have tried increasing my PR and just about every advertising campaign: Adbrite, Google Adwords, Microsoft Adcenter, Yahoo Advertising. Does anyone have any other ideas what worked for me great last time was finding a site targeted torward my users and paying for a Per week link on the sites at that time. Now a days every site ive seen thats relevant is using google adwords. I am interested to see if anyone else has found other places on the net with better results. Any help is definitely appreciated.