Just like there aren't any more natural disasters than there were 100 years ago, we just hear about more of them because it's so much easier to send the news now; and there aren't any more... say... underwater-basket-weavers now than then, they're just easier to find now because they're gathering on facebook and etsy.

1 - Visibility, which you yourself mention. We get to see injustices that we would never see before. Also, like you say, we get to see mob reactions that we would never see before and so we have many more chances to join mobs in a day. 2 - Ease of response. It's easy to post vitriol. It hardly takes any time at all, and you don't even have to walk to someone's house or disrupt your plans for the day. This is a direct result of internet tools that make communication easy, which didn't exist before. 3 - Anonymity, and lack of physical presence. People say things anonymously on the internet that they would never in a million years say to someone in person.

Basically, it's just easier to be in a mob these days...