The Flake tweet absolutely doesn’t hurt Trump. Most will never hear about it. Of those that do, most Trump supporters love it, and most Hillary supporters hope it does damage. The lion’s share of the rest literally could not care less. It is inside baseball to them, and their eyes glaze over.
Otoh, people CAN be influenced by social media. The only people who dismiss Trump’s dominance there are people who avoid social media, or who underestimate its significance. I read a comment recently, ‘I wonder if it burns Hillary that she has to pay people to do what Trump’s supporters do from love.
Whether it burns her or not, she’s at a huge disadvantage in that venue. Social media factors in this campaign as it has in no other, and Trump owns it.
Note, there’s a fundamental difference between maintaining his social media dominance and entertaining. Trump doesn’t tweet to entertain. He tweets to keep his social media activists fired up and on the job. It works.