That said, there's one very important way in which email is superior to those other technologies: it is completely asynchronous. People may send emails hoping for a quick reply, but they generally know better than to expect one. But if you've tweeted recently, people expect quick responses to replies and direct messages, and of course, nothing says "Interrupt me!" like that green light next to your name in someone's IM client. (Whether texting is similarly always-on depends on how old you are, I suppose.)
I haven't figured out quite how to manage all this, and maybe I never will. Typically I set my IM status to "invisible," but I don't want my friends to do the same — if they did, how would I know when to send them a message? So I fall short of the categorical imperative there. Basically, I am coming to realize, I want a medium of communication which allows me to interrupt friends whenever I want to without ever allowing them to interrupt me. I ain't asking for much.