The problem with Facebook is that it does not allow the custom sharing of text, as soon as you share a website it looks at the meta tags of that website and proposes the content around that in your share message.
You can’t send a “custom” text to the box where yo can normally edit your own remarks extra to it. Thus for Facebook it would only end up working as a normal share functionality and not a selection sharer. Or am I interpreting the Facebook documentation wrongly?