Of course he is.
I suppose a medium can't be responsible for its users. I get that as an intellectual proposition. Tens of thousands of incredibly stupid--or even morally repugnant--people have written books, but we don't hold the book responsible as a format.
But for some reason, I think Twitter might be different. It seems to attract more--much, much more--than its share of users who combine vapidity and vanity in ways that are embarrassing even by the standards of the internet.
10 hours ago