In tests I was able to get new users for around $2 each using Facebook ads. I went into the details here:

http://petewarden.typepad.com/searchbrowser/2009/11/get-new-...

In my limited experience it's all about testing and iteration, lots of it. Or just find a feeble excuse to show a picture of a busty girl, judging from most of the ads I see on Facebook.

Also, be prepared for shockingly low CTRs on banner ads like Facebook's. Even highly targeted I was seeing 0.07% on my best ads, and from talking to others that's not uncommon. Even at that low of a rate the ads were cheap enough to be worth running.