I was able to install the Social Zimlet successfully (retrieved v2.1.7 from another forum post, which I believe is newer than what's in the gallery). I saw the new "Social" tab in my UI, and I added my Facebook and Twitter accounts without any problems. All was good... until the next time I logged into the web UI.
Now, I always get this error upon UI login:
... and the "Social" tab is gone.
An error occurred while initializing the zimlet "com_zimbra_social". Please notify administrator.
[Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIXMLHttpRequest.send]" nsresult: "0x80004005 (NS_ERROR_FAILURE)" location: "JS frame :: https://secure.divergentsystems.net/zimbra/ :: anonymous :: line 827" data: no]
I've un-deployed and re-deployed with no success. Perhaps there is something wrong with my user-specific data that's causing this Zimlet to bail, but I don't know enough about Zimlets to know where to look.