I had some similar issues in cms with external links, you have some options, I've tried this two ways:
First
The first thing I've done trying to do this, is to use Link Plugin and insert in the plugin the values needed for facebook sharing. (Not all cms versions works well with this)
Second
When the first solution didn't work for me, I writted the links directly in the django template, because I'm not sure why, but cms sometimes modify the links or plugins I try to add. So I recommend you to create a placeholder before or after the place where you want your link, and write directly in the template the facebook link.
I know these are not beautiful solutions, is the things I've done when I find myself in a similar situation. What I can tell you for sure, is that the problem is CMS. It's CMS who modify the code you put in the placeholder.