As someone who's here but often just lurks, I can offer one explanation - Facebook and Twitter. I spend far more of my NCFC social time on each of those than I do here. Getting on forums and engaging in a deep discussion generally requires you to get on a computer. Yeah, a tablet or a phone gets used from time to time, but if I'm going to bang out more than a few characters (and I'm at home or work), then I'm grabbing a real keyboard.
I think all forms of social media have their place, but there's also been some evolution. I've been online 25 years and seen specific topic social media go from Usenet news groups to email lists to forums to broader platforms like FB/Twitter/Instagram (no, I never did IRC)... Forums are, in all honesty, about a decade past their peak. That doesn't make this a bad place - just a general observation.