There used to be 3000 posts per day at one time!
There are many many more bulletin boards now, and there are also more ways to chat instantly, such as chat rooms that many people prefer.
Also, this board originally began as TalkStargate and all the original members were huge Stargate fans. That's the reason that it is still the largest forum here. But many of them have become disaffected with Stargate in recent years as it has changed.
Also, many members are students with the opportunity to use college computers to access the website. Once they leave they no longer can do that for free, so there is a limited life to their membership.
Another thing is that unless I'm discussing a new film or TV show episode, I personally find that I have probably answered the question before. I'm sure that many people still come and read posts, but don't add a post themselves for the same reason.
If someone does spark a new and interesting thread, you'll find that it quickly gets a response.