Social media are useless for any serious discussion unless you have pseudonymity (HN, reddit) or full anonymity (4chan).
For me, it's got nothing to do with anonymity or some possible later repercussion; rather, it's being sensitive to short-term feedback. Effectively, the bandwidth of human communications through the internet is low and error-prone, and hence stressful for me to get my thoughts across. (It doesn't help that long-form replies -- which help me explain my ideas better -- have been grossly devalued.)
Sadly, it's gotten worse in the last few years -- perhaps due to Facebook, or perhaps due to societal changes that opened the door to mediums like Facebook.
I seem less and less capable of growing that thick skin that people always talk about...
Every tweet is like an underdetermined system of equations with infinitely many solutions (solution == what they really meant when they typed up the tweet, and everything else that readers project onto posters). Everyone has to choose how to interpret ambiguous statements all the time, and by nature our brains are more likely to choose interpretations that conform to our own expectations.
We can create our own private echo chambers if we aren't careful, where we always interpret what people who are on "our side" say in the most charitable way possible, and we always interpret statements from opponents in terms of the biggest strawmen our brains can come up with. This is a pitfall of any sort of communication, but it seems to be a vastly greater danger with online communication.
People tend to blame the lousiness of online communication on other people, and there's some truth to that, but I think it's just inherent in both the medium and human nature. In the long term I hope that digital mediums can evolve to support better communication and provide more fertile ground for more constructive discussions.
When you put it out in public online, it might persist unaltered for years or even decades. My most upvoted comments under an old pseudonym on YouTube caused me to decide to never, ever look up old posts on Usenet.
Online is an entirely different story. The barriers to communicating efficiently makes people default to pattern-matching instead of thought, which pretty quickly devolves into the lowest common denominator of conversation.
As you say, places like reddit are better for this, where people have a higher tolerance for longer conversation and the interface supports longer posts. I'm part of a subreddit that's probably in my top 3 fora ever for quality of conversation ever, but it relies heavily on the fact that the norms are pretty well-internalized by everyone.
Sure, the site administrator, your ISP, or the NSA may know it's you posting, but the other users of the site don't. And you don't have a site wide identity that follows you around. Unless you explicitly choose so.