Why doesn't SA actually post the comment one has made when you want us to push to LinkedIn or Facebook or Twitter ? I can understand with the Twitter limitation but the other platforms?

The comment the reader makes should get posted to their social media platforms where possible.  Right now it just says you commented and the link.   It's not going to take away any clicks to the article which link is showing.  It just helps frame it as it is going to our individual social platforms and helping promote linkbacks to the SA article.