The cliche was always that 'everybody's a critic,' but it becomes truer every day. Long before reviews appear in the traditional outlets, you can now usually discover - somewhere in the thickets of the Internet - reactions to shows from people who've seen them in previews.
I write the occasional entry for the 'Times' Theatre blog, especially when I'm in London and seeing two shows a day, but I don't tweet. I don't want to have to express my opinion in 140 characters. That's like writing haiku. You need a certain amount of legroom to review a play properly.