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.
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.