Ok well, I'm thinking their should be a link right next to where your logged in, that says "Notifications." If someone has sent you a friend request, posted on their dashboard (maybe not this, since it is very comm), or PMed you, it would be highlighted a color like white, so that you knew something new happened on your profile. This way you would know to check it.
What we'd probably do is introduce a way to notify you that "you have unread messages", and then not Email you again until the next time you log in. That way you don't get spammed.
