Originally, tweet length was 160 minus username length. One day, I turned to Jack and pointed out that I got one more character than him. He said, "That's not fair. We should standardize." I agreed. He said, "How about 140?" I said, "Sure." And he updated the code.
Last year, we upgraded tweet length to 280 and guess what? Use of 'please' is up 54% and 'thank you' is up 22%. I love that! Also, people are asking more questions and having more conversations. All this, and the majority of tweets are still under 140. It worked.