I found out this very early, a few years ago when their ad platform was barely launching for everyone. I had a digital magazine and overall it seemed like a good deal, send me people interested in music from these countries at very low CPCs, nice. In total, I may have spent around $1200 USD over a month for a little more than 100 subscribers; needless to say I felt scammed. When trying to figure out what happened (w/ Google Analytics, my own event tracking code and even a few apache logs) I found out very interesting stuff, like that 99% of those clicks were people who didn't stay for more than a second on my webpage, like in: they didn't even wait for the page to load completely... weird.

Just for the record, my ads were not clickbaity and were targeted fine, and my magazine didn't suck (IMO at least heh), so if someone clicked on my ad I would pretty much expect it to stay in the page and have at least a look at the cover of the current issue.

Why did I burn $1200 if the ads weren't working from the start? I wish I had figured that out earlier and spend that money on a fancy chair or whatever instead... The thing is that I was only looking at my daily visitors and believed that everything was fine, it was until the end of the month (where I always made some kind of audit thing to see if I was growing or not) where I noticed that the number of new subscribers had remained the same even though FB ads were active all time.

Since then, the only advice I give to friends and clients is "stay away from Facebook ads, it's not what you think they are". And on a small side note, I tried a lot of advertisers at that time and the best experience I had was with StumbleUpon, their referrals converted to subscribers at an incredible ratio (like >50%!!!) and on top of that they drove some extra organic growth even a few months after then campaign was finished, respect for them.