Netscape: Vanishing bookmarks

OK, I'm stumped. Half of my bookmarks in Netscape just vanished. I'm not sure exactly when this happened, but I just went to find one now and a good chunk are gone. I have a backup, but it's fairly old. I know there's a bookmarks.bak file (or similar); how often is that updated generally (do I have a hope at recovering what went poof)?

did you accidentally switch your user profile?