It's even profitable, for crying out loud.
Whether this is a good decision and price for Facebook is to be determined. Many thought Google overpaid for Youtube and FB overpaid for Instagram, and look how those turned out. In addition Zuckerberg likely thought that much like losing image sharing, losing messaging would pose an existential threat to Facebook.
Zuck's strategic track record has been stellar, all the way back to the beginnings of Facebook. The one "mistake" he admitted to (HTML5 vs native) was corrected promptly and thoroughly. I'm willing to bet he's got a much better handle on these things than given credit for. I'm long on FB and buying on any dips.