I always figured it's the outdated forum software. I have display hickups of all kind of sorts. Having to block weird imbedded scripts doesn't help with this, but I have similar problems with G3 (no Emailnotifications there, either).
Why for god's sake does SHS link to facebook.net? Is this really necessary?
Even when the software was current, I didn't get email notifications from the forums--it seems like users get everything from the forums (e.g. Argent77) or nothing (you and me). I remember looking in the code when I first started noticing it, but I don't recall seeing anything in its email subroutines that would make me think it's a software issue.
Though I have nothing concrete, I suspect that some upstream provider has decided that IPB forum email--or maybe G3/SHS specifically--is spam and binning it.