Ad Builder now offers the ability to create ads from an existing Facebook Page Post. To utilize this feature:
1. Navigate to your Facebook page and click on the timestamp of the post you'd like to use in your ad.
2. Copy the ad post id from the URL.
3. In Ad Builder create an ad as you normally would. When you get to the "Creative" tab, choose your Facebook Page and select "Post"
4. Enter the Post ID you copied in step 2.
5. Complete the Ad Builder ad template creation process as you normally would.
You may also use the same method to launch existing ads using the Ad Id.