Just started evaluating Zimbra a few days ago, both for myself and for some of my customers.
Server end works great. I've been playing with the Desktop client (by the way, is it a real desktop client or is it just showing web browser pages under the covers?) which is quite nice although a little slower than Thunderbird.
Anyhow, discovered the social tab and added access for my facebook and twitter accounts. They seemed to work fine until I quit the Zimbra Desktop and restarted it at which point those added accounts were gone.
This is totally reproducible, I tried it a few times.
Running Zimbra client on a Mac