The reason social media sites are no substitute for letters is that in a letter you don’t usually keep a running total of how many times you picked your nose in a day or other trivialities absolutely nobody cares about, including the minutiae of your little daily thoughts. In a letter you learn to write what matters whether it’s information or real insights. That’s the opposite of what Twitter for example is training people to do. So even if people do “write” on social media, it’s in no way the equivalent of writing on paper.