I see newcomers come and go on this site and forums. They register, post a bit, and then don't blossom into active members. Why is this?

I don't understand it. I was on a forum that had 100 members active everyday, 7 days a week. Their news and the topics they covered in the news was narrower than PSU, yet the forum and site was very active.

Then, we've got old members leaving and thus less active members to keep the forums going.

Is there any way I can help or something we can do to help make newcomers "stick" to this site?