If traffic is the only thing that interests you, then it's fine. But if you are looking for a smaller Bounce Rate, a longer time spent on site, bigger sales, and so on, that is definitely not the answer.

Be careful with that -- Facebook is indeed cheap advertising, but it tends to convert very poorly for services charging money.

So, while yes, it might be 'cheap' traffic, that doesn't necessarily make it cost effective.

If you're attempting to just get eyeballs, then by all means, it's a great way to potentially get shared around. If you're hoping to actually get sales out of your ad spend, then AdSense is usually the safer bet.

As with all things, your mileage may vary.