Most forums have fewer active members and less posting these days, it's not just this one. Social media has hit forums very hard, as when experienced gamers stop playing or take breaks, there are fewer and fewer younger and newer players to take their place on forums, because they are all using Facebook, Twitter, You Tube, and blogs.
Speaking for myself, I've always preferred forums, but I don't log in here as much as I used to because I don't really play Age of Sigmar. I did play a game and enjoyed certain aspects of it, but I didn't like not having ranked units any more. It took forever to move large units and it discouraged me from playing any more games. The Age of Sigmar approach has worked much more effectively on 40K, and I've seen an increase in activity from members on 40K Online since eighth edition came out.
The biggest turn off relating to Age of Sigmar for me though is the narrative. It's simply awful. It's vague, uninspiring, and feels very bland. Also, the Old World map and narrative were so rich and full of depth, and I am still upset that they have been jettisoned and replaced with something so superficial and poor by comparison.
This forum was designed around the Old World Vampire Counts. It is, therefore, unsurprising that activity has suffered. There's really not much for members here to get their teeth into (pun intended) any more.