I started using Facebook ads in 2009, and always noticed that Facebook Ads stats showed very large number of clicks, both in their CPC and CPM model, whereas actual clicks recorded on my websites were significantly lower.
My first point of catching incoming traffic was my own script, which I had developed in PHP. However, for more detailed info I always used Piwik as my main Analytics software, and also used Google Analytics for some of my websites.
Results on the above three were always almost the same, and much lower than what FB stats would show.
Recently in 2012 BBC published an interesting article, where they investigated the truth behind Facebook clicks and ‘Likes’ and how Facebook has been cheating in order to grab money out of pockets of its advertisers. What they (and many other investigators too) have found is that Facebook has automated bots setup to click on your ads.
If you search the Internet for Facebook Ad Fraud, you’ll see tons of information on this fraud. I found another good link here.
However, it remains a mystery why Facebook hasn’t been sued for this cheating. Seems like this cheating is still legal. Apparently if you setup bots to click on your user’s ads, then it is not considered fraud. After all advertisers are paying for the clicks. Who clicks their ads, is irrelevant.