I tried ads on facebook mobile for a couple of my ios and android games and, while the ads reached several hundred people, in the end the cost per acquisition was (the sort of industry standard) $1 per install. Considering my games are free to download, monetize only via ads and every player provides, on average, a few cents, it didn't seem like a good strategy to pursue.
On the other hand, a short campaign for "likes" on my company's page was very fruitful: FB was predicting between 7-27 likes per day and I got almost 100/day instead!
Using twitter doesn't seem that effective in terms of installs for me either (and I currently have more than 2,300 followers, almost all game related). How do you move there? Any particular strategy?