alx once posts are read and don't contain any new message, they move down like any other discussion for logged in users.
So even if you have a big number of pinned discussions, users who read them (or clicked mark all as read, I think) will no longer see them at the top of the homepage until someone posts in the pinned discussion again.
Pinned discussions are always at the top when you visit a tag page, even if you read it before.
