facebook ads are for the most part very similar to pay per click advertising. You can opt to pay per click or you can pay per 1000 impressions, etc.
I ran a campaign for a number of weeks and I personally thought it was genius marketing for one simple fact: Facebook allows you to set geographic targets, such as knoxville, tn .... and then allows you to choose how far from that center you want to advertise, perhaps only 10 miles, or as many as 100. Only fb members that reside within that demographic circle will see your ad. For certain campaigns that need local exposure only, this is priceless. I was selling an idea rather than a product or service, and I did have some interested locals show up on my website from the fb ads.
On a national or worldwide level, however, I have no experience with it.