I guess one reason might be that they have a pre-existing social network to draw from, so much easier to coordinate social VR with friends. Not saying that is

I guess one reason might be that they have a pre-existing social network to draw from, so much easier to coordinate social VR with friends. Not saying that is sufficient, but it definitely helps compared with a startup that has to rely more on having strangers interact or, if meeting with friends, is likely to be higher friction that FB can offer.