Watching the X Factor tonight, I was struck by a couple of things; Simon Cowell was right on the money with all his comments; the crowd booed whenever he made a critical comment. To my mind, the acts needed to hear his comments, so that they knew what to do to improve. By contrast, the other judges, Dannii Minogue and Cheryl Cole, found it difficult to say anything other than "you were brilliant".
What's the link to social media? It's to do with feedback. In my opinion, far too many comments about blogs or articles fall into the Minogue/Cole category, telling the author "you are so clever" or "I agree with everything you have said". That's polite, but not helpful to the original poster, or subsequent readers.
Of course, there's no need to be rude, or to resort to personal abuse. Simon never does that. Instead, say why you disagree, and don't be afraid to be critical. That's what good honest debate is about. Though it's nice to get people telling you how great you are, it doesn't help you to improve one bit.
So take a leaf out of Simon Cowell's book. Be direct, be honest, and tell people how they can improve.