I am so incensed at something I have read and would like to comment in the space available when I share the news and link.

It is almost always a mistake to comment on an article you are sharing.  If the article is part of an ongoing debate and you feel the contrary view should be shared,  then find one that articulates the view you support and post it.  

By commenting yourself you are unlikely to change peoples views and are more likely to harden the conviction of those with a view opposite to yours.