It's surprising when people find it difficult to understand the importance of email/some kind of notification for the activity on your social profile, but the this cannot be the only reason to be credited with the failure of Wave. I still feel that Wave is a great product and it could not catch up just because it was ahead of its time. And the few admirers could not use it because others were not ready to use it. A lot of new age collaboration tools do the same what Wave did long time ago.