No matter how much searching I do (internal MP search, google search of all of MP, etc) I can't find some old threads where I posted some info I'd like to retrieve. They certainly weren't deleted or closed by any mods.

I also notice that my post count never really increases.

Does MP delete old threads to keep the DB size down? Or for any other reason?

No, we do not.

Go to your profile page and click on contributions. You should be able to find what you are looking for.

Slight correction: we do delete old "For Sale" posts.