I've been on here for quite a while and I think the reasons people drop off are varied. Some folks sold their cars, maybe didn't talk about it, and just kind of slunk, slinked or slank off. Other folks got busy with house, kids, wife, NO wife, golf, etc. Technology issues might be at play - I know the ads are giving some people trouble, and the mobile implementation is less-than-ideal. Then you have the people who just don't click with one aspect or another of the site: the current frequent posters, the moderation, the politics, the banned topics, etc. People participate where they are comfortable for the most part. As the site and the membership has changed over time it moved some people to check out. I don't think it's one thing - people come and go for their own reasons. Personally, I hate it when one person pollutes almost every thread (flooding) and when I see that happening on a forum, I'm out.