IN simple way: When you post a normal post in fb it goes as usual. But at some times you want it to be boosted to more audience for page likes or website conversions. Then this dark or unpublished post comes in the picture. Often, you aren't allowed to boost that post as because of this message- more texts in your image . Now instead of publishing the same post over and again after removing text from it, you make a dark post which will not be seen to your followers but will only be shown to the people whom you are targeting (they may be followers or non followers of your page)
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