OK, here's something that I have tested in the past: Choosing to promote Facebook ad sets for a bit longer (with the same lifetime budget) to see if it helps reduce the cost per engagement and /or view. 

Here's the theory: when you tell Facebook (or any ad platform for that matter) that here's $X and here are my limited dates, I need to achieve my ad objectives. GO! Facebook understands it as a matter of urgency...think about it as something that needs to communicated urgently to the target audience before it's too late...instead, what if you added a few extra days (where permitted) and then checked if the CPV and CPE did in fact decrease...wouldn't you be slightly happier?!

Of course, correlation doesn't necessarily imply causation. But if you've been running FB ads for your brand, you would have a good idea about which ad sets to exclude (ad sets where targeting is unusual, contests, buying models are different). Once you've removed the data outliers, customize the columns in Facebook ad manager to include 'Start' and 'Ends' dates from the Settings tab. 

Do a simple excel formula to calculate the number of days the ad set was promoted for (End - beginning date + 1) and you now have number of promotion days on the X axis and cost per view/engagement on the Y axis. Right click anywhere on the data and Add Trendine along with the linear equation and  you now know if there's a negative relationship between days promoted and cost per engagement /view - which is something great to know.

This approach might probably work best with always-on content where you can know for sure that the content has remained more or less the same (to compare).

What do you think of this approach? Something that you'd like to test?