Pretty good thread on why XMPP failed but SMTP didn't:
Discussion
I used to use Trillian. I had a lot of accounts on it: AIM, ICQ, MSN, Yahoo, etc. Those times were better. I don't know why it went away.
I tried to use it recently and found the popular software far too complex, both the server and client, and also far too limited. Not a good combination.
Xmpp definitely still used