I checked my Yahoo account and there is nothing in my spam folder for the entire last week -- during that week, I should have had about 25-30 notification emails from Facebook.
I did check my Facebook settings a few times this week and my settings are OK. I also wrote to Facebook a few days ago and checked their FAQ help page, but the problem doesn't seem to be Facebook.
So if the problem isn't FB, and it's not my Yahoo spam filters, I thought maybe the problem was with AVAST -- these emails stopped coming at the same time I upgraded my AVAST. Is that possible??