It seems like the Facebook algorithm will really limit the reach of your post if it features an external link. Exceptions are when you share an article from a well-known news source.
It is not as obvious as the limits it puts on posts featuring YouTube videos versus videos uploaded directly to Facebook. Still, it seems to be an issue.
In part, this is a result of Facebook trying to crack down on the fake news mess of the last year. Also, Facebook does not want you to direct people away from Facebook.
So if you have to feature an external link in your post, ideally you should put some money behind the post, targeting followers of your page or one of your custom audiences. - Cris