Facebook spams your inbox. They make sure you know that you have an account and that people are trying to reach you (or tell the world about their cat, etc).
With wave, it's silent. There was activity on a wave I was following in January. I found out about it in March. Someone replied to something I said on July 21st. I'm only just finding out now. What kind of a communication tool is that? It's like they're selling a phone that can't ring.
The other UI and social issues were important but weren't fatal. If Google had actually notified users when they got new messages, they would have logged into wave, sent more messages and wave would have lasted longer.
Instead, messages were sent but never read, so Wave lost all the momentum it had.