In theory, a FB ad campaign sounds like it should be great. You can control a lot of demographic variables for who sees it so you can really drill down. I was underwhelmed though. I ran a campaign last spring and while I picked up 40 or so likes, none of them converted and I don't think any of them ever even interacted with any of my posts. Still, I don't think I spent more than $125 if I recall, so if you have a newer page and you're looking to boost likes simply to add legitimacy to it, it may not be a bad way to go.
I agree with Tony on Twitter. You really have to work it to see any sort of result. If you do nothing but log on, tweet "I DO LANDSCAPING!" and nothing else, you may as well not even bother. You really have to engage with people, become part of the conversation, and treat it like networking. It's hard, and I put significant effort into it for a while and didn't land any business from it. Now that said, I did make some great connections with other professionals, but those pretty much poofed when I stopped tweeting. I'm not sure how seriously I'll retry twitter this year.
Anyone seeing success with LinkedIn?