Various reasons in no particular order:
* Internet is built for the fast and not all of us are there yet. Compared to ten years ago dial-up worked just fine. I don't go to websites for tv stations, tv shows, movies, or anything like that because some flash and other long-loading shit is built in so it's not worth it. For the percentage still on dial-up, we'd rather shut the computer off and do something else. Either that or let the entire thing load but go off and do something else for a half hour while it does, so we go to less places online as a result.
* Not as addicted to the internet. I remember one time when I was about 24 or so and I stayed up for a day and half on the fucking internet. Ew.
* Yes, it's bigger. More to do so the pool of people spread out. Remember when ezBoard was the thing for most of us? The options have increased exponentially since then. People got into social networking, did their own boards, or quit boards and only do stuff on IM or something.
* Not to depress anyone, but it might also be dead because people around our ages start popping out critters and working on careers. So our particular pool of people thins out. If you go to where the stupid kids are, like Twilight message boards or some slut's YouTube account or some spoiled brat's facebook, I'm sure you'll find plenty of activity.
I don't mind adding family (well, extended family) to my MySpace, but some of them keep deleting or changing their accounts. It's irritating. What the fuck are you doing that makes you delete the thing? No one's in a high profile career or a business person or something. Edit that shit out and move on. And then you have other people that are just trashy. Ew.