Facebook is a social network, integrated in our everyday lives. We browse it while we ride the commuter train, during dinner, while watching TV, in the toilet and when we are out in the nature.
Of course, some of us are more interactive, sharing stories and news while others are more passive, watching and liking. Some of us use it as a tool of communication, while others just check it a few times a week for entertainment value.
A VR headset, in all its glory, still takes a lot of computing power, heavy equipment and segregation from the "real" visual inputs of the world.
You are unlikely to whip out a VR headset, a computer and sensors out at a cafe, and even if there was a mobile version, you're still out in public, endangering your safety by blinding yourself. But while out, you are likely to sit in the sun, have a cup of coffee and put on a pair of sunglasses. Which is not a far stretch from using AR.
Imagine a mom, driving a stroller, trying to check her grocery list and a conversation with her mother on FB.. Instead of stopping every few minutes to check her phone so she won't run the stroller in to traffic, she could have the information integrated in to her field of vision.
In short. FB VR will be a computer based, sit down type of venture with social VR rooms and games, while FBVR will be the mobile version, with simpler functions that display the necessary information from ones feed, steered most likely by a smartwatch and voice commands.
FB AR will be a revolutionary step for FB since it will not only drive the AR market forward, it will make lives and information sharing simpler and more adapted to the flow of our everyday lives. No more heads down our phones, both hands free.
Sure, we'll have to learn to adapt our attention, and to design things that won't overload our cognitive processes but that is a post all on its own.