I tried advertising with Facebook for a while, but actually found that I was getting a better deal with Google Adwords. PPC advertising with Adwords is extremely targeted. If I have a dating site in New York, for example, I can have it so that visitors only see my ad when they are looking for "New York Dating" or "Dating in New York"....you can't get much more targeted than that.

Promoting your site by posting in related forums is definitely a good idea. However, as you see more mentioned DeeEmm, this can be very time consuming. You actual have to build relationships with people and get noticed for your meaningful contributions...blah...blah...blah. I've actually been banned from a lot of dating related forums. Some of them I spent weeks contributing in a meaningful and intelligent way. Then when I finally made a mention to my own dating site....BOOM! You're Gone! It can be very frustrating. But I guess it really depends on your niche and the time you are willing to spend.