The Facebook phone flopped, or so I assume since I haven't heard about it in over a year. Think back to when it came out: Microsoft had the Live accounts back then, obviously there are your Google accounts for Droids, Apple iCloud accounts for iPhones, etc. If Facebook had more penetration into the cell phone market, maybe the idea of a Facebook-central account that starts off as your email makes more sense? Again, I knew little about the Facebook phone, so just conjecture.
Buying WhatsApp seems to be a sign they're steering the ship in a slightly different direction, moving away from any hopes of the above.