I guess it's because there's an exodus of people leaving? I mean, I've barely been here 3 months but from what people have said, they feel like the MB isn't what it used to be; whether the general vibe, the people, topics discussed, I don't know. Many people that have been here since the beginning are.. growing out of it, moving on. It's an aging population here. When I got on here, I expected to be waaaay older than everyone else, but for the vast majority, I'm still a little older, but only by a couple years.
Old friends have left and aren't coming back, and some people don't feel like they've connected with the newer generations.
There's nothing that really involves the entire community. I have yet to see a forum-wide rp, but that's just me. And yes, the environment can get a little toxic sometimes, but that's what you get for talking with other nerdy teenagers on the internet.
Personally I wouldn't say the SAMB is "dying" or "fading away" but maybe just going through a lull. People are going to leave, but new people will come. And maybe they can't replace the great MBers of old, but it's a start.