The second Facebook Ads Experiment that you can run is Holdout Testing.
Holdout Testing uses a holdout group to measure the total conversions caused by your ads and looks at Conversion Lift
A Conversion Lift test compares people who had the opportunity to see your Facebook ads with those in a holdout group who were intentionally withheld from seeing your advertising. The incremental effect your Facebook ads had on conversion activity, such as purchases and other standard and custom events, can then be measured. You can run a holdout test on a specific Facebook ad campaign or across all of your Facebook advertising.