I have the same sort of reaction to Piazz(z)a, and I think they try the solution you propose. I think it's because I perceive Piazza as encouraging the course staff to send more emails than they otherwise would. This may or may not be true, but I subconsciously think it is.

It _is_ true that I get more random invitations to things I only slightly care about via social media, and those rare invitations I get by actual personal email tend to be things I strongly care about. So the argument can be made that Facebook and G+ _are_ spamming me, by encouraging my acquaintances to spam me.