Around 9 days ago there were a couple of interesting questions added. Both of these got a reply (from me, in this case), yet looking at the profiles of both users they've never logged on since their initial posting of the question.
Is there a way to suggest that users subscribe to email notifications for when their question is commented on and/or answered? I assume it's not a default (is it even available?).
It's quite possible that they lost the address of the site (it's easy to forget or get it mixed up). If I remember right there's no welcome email either, so nothing for them to look at to find their way back.