"You don't just throw your letters in the trash. You might want them some day." Maybe it's just me, but I actually /do/ throw my letters in the trash. I /do/ treat Twitter, etc. as ephemeral and passing. I don't care about saving those messages. Am I the only one?
It's not just you. My fear is opposite to the one the author describes: That by using these services, all these messages that I treat as ephemeral are actually being recorded forever, and 20+ years from now something I said offhand to a friend or a stranger will be used to embarrass or discredit me.
Even if no-one needs to discredit you, an entire history of you will always have value, and companies like Google, Apple and Facebook will have unique access to it.