The main reason for the decline in teh acitivity in this forum I think is the fact that most of the very active members have left Renderosity, I name "wabe" and "rutra" as examples. Walther Beck aka "wabe" left and the Vue FaceBook group got started, and I joined soon after. But it's not working as a forum as such right now, more like discussions and "see me and my work" kind of thing (which includes my activities). The active mebers of the FaceBook group are sort of the hard core Vue'ers ("the usual suspects") and the pictures uploaded there reflect that. But a lot of them get posted at C3D and here as well (mine do, so far).
Don't expect the FB group to replace the forums e.g. here and at C3D.