Facebook executed very well on mobile after their post-IPO slump, while Twitter failed to create an easy onboarding experience that would enable them to move beyond their early adopters (the tech industry, the media industry, and celebrities). The Twitter executives were too busy squabbling to move the product forward (see the book Hatching Twitter).

How Facebook Squashed Twitter: https://stratechery.com/2016/how-facebook-squashed-twitter/

The Reality of Missing Out: https://stratechery.com/2016/the-reality-of-missing-out/

The purpose it was made for has become irrelevant. Everybody got smart phones, so the SMS character limit became moot, as well as the need for a lightweight interface. Everybody got Facebook, which satisfies all the text based posts people make. Everybody also got instagram, which is a much better image platform than twitter could hope to be.

The exception to all this is Twitter bots (but not MS' Tay), which are probably the most entertaining, unique, and valuable accounts that could not be mirrored on another site with the same effectiveness.

They did not make many missteps. Once the general public started using Twitter by 2013, they realized it's not for them, unless they were a celebrity.