That last line is probably the one thing I love about facebook, the "5 years ago today" posts that show you little snippets of your past activity. You probably wouldn't go and actively look for them, but seeing them is always a nice surprise!
A while ago I built basically what you describe but via whatsapp instead of email [1] and I think those are the perfect interfaces for filling out a journal, so many people recommend writing "as if you were writing to a friend" and that makes it so much more natural than any app or website could.