I can't find the thread anymore but someone around here used it to promote his trading card game (similar to Magic: The Gathering I think) and found it very efficient.
That would be me, and you can find my DevLog
here. I can't say Facebook advertising has been 'very efficient', because it looks like hits from Facebook may be converting into sales at a lower rate than other sources of traffic to my site. It's actually quite hard to connect sales to their source, unlike hits and demo downloads which I can measure quite accurately.
If you get a free coupon I definitely recommend you use it. Bid lower than the recommended levels, as it's better to have a few hits per day that make a profit than hundreds of hits which cost you on average! Also pay a lot of attention to which regions you're advertising in as well as the message itself, and remember that conversions are your goal, not click through rate (which is Facebook's goal).
I should probably write another post about this stuff on the DevLog!
if possible make a landing page specific to that ad, so you can precisely know how many of the people who bought the game found it through that ad.
Alternatively you can specify ?utm_source=facebook&utm_medium=cpc&utm_campaign=[whatever you like] on the end of your URL. Then you should be able to identify this traffic in Google Analytics.