It's true, people remember about the site and forgot the next day. But also remember, a lot of people that don't post simply lurk. If they posted the forums would be a lot better/more active/fun but having them lurk the forums still helps a little bit.

I normally don't post that I'm back for that reason.
But yeah life comes at you fast and you'll find your self away from forums for days or months, just to pop back in for a couple days because you interested in a topic or wanna see about a new game, then it's back to life. I use to love spending all my time on forums learning about stuff and I still do but the amount of time we have in our free time can be an issue as we grow up.