When you're running Facebook ads, it's crucial to pay attention to the evolution of their relevance score, which you get after reaching 500 impressions per creative. Here's the official definition given by Facebook.
Basically the better score you have the less you'll pay per click or impression, since Facebook naturally favours ads which please their audience. The relevance score is influenced by the reactions on your creative (likes & co) ,the engagement (comments, shares) and also, potentially, people asking Facebook to hide the advert (because it's not relevant to them or simply annoying).
I'm always trying to keep the relevance score of my adverts above 7, ideally 8.
Beware that the score will usually drop after a few days, due to the inevitable advertising fatigue. You should also check another important piece of data: the distribution frequency of your creative.
Once you go beyond 2 times per user (i.e. each person saw your ad at least twice), you should run another creative, otherwise you'll see your relevance score drop very quickly, below 5 (and your cost per engagement will increase accordingly).