Some of the reasons Facebook took away from email are UI/UX-based: 1) have you ever wanted to send just a message without a subject line (email forces you to st

Some of the reasons Facebook took away from email are UI/UX-based: 1) have you ever wanted to send just a message without a subject line (email forces you to stop and think for 2 seconds, which is not good), 2) you have to remember the person's email address or how it starts (emails are still not great at just entering the person's name), 3) if the person changes email address you have to know the new one or your mail goes undelivered. FB gave these folks email without all the above extra and folders, tags, and all that other non sense.