He's not denying that it has. He's refuting his former naive belief that open communication would be some kind of magical pixie dust that would make better everything it touched.

I can relate to this. I remember thinking, back in the 90s, "once people can directly talk to people from other cultures, and see each other's common humanity, how will there ever be any more war?"

Turns out that was an extremely naive belief. Just the opposite is true in many cases: direct communication can aggravate differences. People's self-righteousness tends to escalate when they are directly exposed to their ideological foes.

Inconclusive. 'A lot of good' means that something is clearly a net positive. In the case of twitter that would require the weighing of a large pile of issues that twitter heavily influenced where one side of the issue thought twitter was just great and the other that it was terrible depending on the outcome and who you were asking.

There is no absolute standard for 'a lot of good'.