I don't use facebook, but couldn't that also be an optimization? Like don't send the user email notifications if they're already on the site where they can see the # of notifications?
Then, once I'm away from the computer for 30-60 seconds or so (I'm sure they have a system training for this), I get the email notification -- even if the original back-and-forth occurred an hour ago. The timing of when I get the email has nothing at all to do with when the event occurred. It's completely dependent on my interaction with Facebook. Many times, it takes so long, I'm usually thinking "Wow! I wonder what else X had to say?" but I click over and it's the same damned thing I already interacted with.
I've never got a later email about something that happened live while I was browsing the site, and was already notified about.
So you're saying that there's a bug. And the bug waits for me to stop using Facebook, then emails me things from Facebook.
I find this quite difficult to believe. At the least, it's a very convenient bug.
The bug would be to get emailed about things that happened while you were using the site and were notified in real time about already.