Social media is just communication. Behind communication lies people. People are good and bad. Mostly they are good. Or so I like to believe. I do think though, and hope, that social media will, through time, bring out the greater inherent good in people. Unfortunately there are those who think not so well before they type - and I am one. That brings me to my other thought. A second good gauge is to ask oneself "Would I be happy saying this in front of someone to their face?".
Having fallen foul of my own advice on a few occasions I do pull myself up more often now than not and ask myself that closing question. What do you use?