One BILLION people just got their messages encrypted. Facebook was under no obligation to do this; for the vast majority of tech history the messages sent by 99% of people were very insecure, and when the tech giants responsible were asked their response was "ehh". This suddenly cuts into a huge portion of that. Pretty much everyone with a mobile phone in Europe or South America, as well as large parts of Asia, suddenly now has completely encrypted messages.
Kudos to Whatsapp for this fantastic move.
On a related note, if anyone is inspired by this announcement to start using encryption in other parts of their life, I have a handful of Keybase invites available (to bypass the 25k+ waiting list.) Keybase's security depends on lots of people tracking other people, so only ask for one if you'll track other people. I see too many people that make an account and nobody ever tracks them/they don't track anyone. My email's in my profile.