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.
The content matters too. They are certainly spamming you if the email they send just says "<Person You Know> just sent you a message on <Service>!" and force you to log in to read the actual message. Just put it in the email, willya?
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.