sw007 - there are hundreds of millions of people on FB - plenty of market share to work with. The app itself is viral in nature - get your friends using it. IF it is GOOD, it will spread naturally. No reason to give up.
From what I see (and this was mentioned elsewhere in the thread), you're going to have trouble building that viral aspect without the self-propagating behavior. If you're not emailing, contacting, or posting on the wall until there's an actual match, there's no reason why the matchee might end up on your site.
Apps that have some amazingly useful quality get talked about. Something like this probably won't be mentioned, even by the users, as much, because it's more of a short-term fascination than a must-recommend tool. At the very least, do you ask the person with the interest if they want to post a status update / wall notification about the site?