Recently I've been thinking about whether it would be better to pay cpm or cpc on Facebook. I've advertised with cpm in the past and have never gotten a CTR greater than .004% or .005% on Facebook. However, when I've advertised using cpc my CTR was 10% higher at 0.05 or 0.06%. I believe that Facebook intentionally places cpm ads in places where they know the ads won't get a high CTR. Additionally, I also believe that Facebook wants to place the cpc ads in areas on the site with high CTR because they want to make more money off people who are only paying for clicks. Conversely, if you're paying for impressions (cpm), Facebook will make money regardless of how bad your CTR is because you're not paying for clicks but rather impressions which is why I believe they place cpm ads in areas on the site that get lower click through rates. This is the reason why I don't think anyone should ever waste their money on cpm ads.