Things here have really changed here over the years and I always wish that people could see it back when I first joined, because this place was hopping! It became my substitute for TV and most other forms of entertainment, and I was here all the time.
I have to wonder if part of the drop-off is just the fact that there are SO many other choices out there now. I've lost touch with a good number of people, including many I've met here, because it seems like everyone has their own favorite site or app they'll ask me to join in order to reach them. I try to resist joining gazillions of social media sites--not because the people aren't worth it, but it's just that even if I did join every site that was mentioned, I'd never have the time to keep up with them anyway.
So, I generally choose to keep in touch via good old email (which I realize has also gone the way of the dinosaurs) and am very thankful for the few people out there who are willing to still keep in touch that way.
I also have to wonder if most people have lost general interest in forums these days, because it does take a bit of effort and time to write out one's thoughts, and everything now days seems geared towards a 3-letter abbreviated text.
I would guess that many people don't feel they have the time or even ability in a forum (it's amazing how hard it can be to translate thoughts into written words.)
I also think that the amount of criticism on the internet seems to be ever-increasing, so it often seems like many people want to read what others write, but are afraid to add their own thoughts after seeing how much negative feedback others get.