@polychromata Point of order: They have joined the Social Community Group, not a working group. The only way to update the ActivityPub spec would be for a working group chartered for the purpose to do so, and no working group has been chartered.
(Community Groups are not allowed to publish W3C recommendations)
That said, someone could decide to try and form a W3C working group for that purpose, and I'm not sure how well that would go given that the W3C is relatively friendly to its members (of which Facebook is one); the previous Social WG being an oddball in having so many invited experts