When Facebook released email, I were quite sure it would be super popular. But this article tells quite well, why nobody wanted to actually use it. I wonder why they didn't fix these obvious flaws. I have some friends who aren't using any other email, so Facebooks (even bad email) was still a great option to reach them by email.
You know people who _only_ used Facebook e-mail?? Never attaching files or adding CC?
Although, if Whatsapp acquisition tells us anything, is that Facebook is not king in the realm of messaging, be it e-mail, inbox, SMS or anything ;)
What it tells us is that most companies who are successful normally only have one hit not multiple. What they do have is the money to buy other successes :)