What is Null on Facebook – Null Meaning on Facebook

What is Null on Facebook? On Facebook my friends have been putting null as the description for their picture, what does null mean? You will see a null value (—) when a given metric does not apply to your ad. A metric may not apply to your ad if: The selected metric does not correspond to the ad format or objective. For example, you may post an ad which only contains an image and no video, but but you selected the average video watch time metric.