Dick Costolo in a Bloomberg interview in May[1], when asked what he thinks of skeptics who say he should be fired:

https://www.youtube.com/watch?v=41NrVg-7bQw

[1] Twitter CEO Dick Costolo Q&A: What to Do When People Say ‘You Suck’

http://www.bloomberg.com/news/articles/2015-05-07/twitter-ce...