http://www.wpbeginner.com/beginners-guide/how-to-automatically-post-to-facebook-from-wordpress/

There are three ways listed:
1) Using IFTTT this is probably the best way, it is free and unlimited
2) Using buffer, which is NOT free.
3) Using a wordpress plugin – which for the free version is Jetpack

The problem is getting the tags right
I’m not absolutely clear how to do this in the free version, it has to be tied up with the Post Format and what, why and how    I wonder if all themes use these?

https://nathaningram.com/when-facebook-isnt-using-the-featured-image-for-your-wordpress-post/