Video tour of the basic Social Zimlet options is available here: 
Enable/Disable the Social Zimlet
Typically the Social Zimlet is enabled for each user by default. If a user would like to disable the Social Zimlet, they can access the Preferences tab, select Zimlets, and then uncheck the Social Zimlet. When Zimbra relaods, the Socialize and Social buttons will no longer appear.
Social Zimlet Preferences
Hiding the Default Feeds
The Social Zimlet by default pulls in and displays three feeds. It displays TweetMeme's Most Popular, top Twitter Trends, and digg's 'Popular in 24 hours'. To modify the default feeds so that none of these display in the main page, click on Preferences at the bottom left. Uncheck the top three options and click on OK.
Adding a Twitter Account to the Social Zimlet
To Add a Twitter Account to the Social Zimlet, click on Add/Remove Accounts at the top left. Click on the button to Add Twitter Account and follow the instructions to login to Twitter, retrieve a pin number, and authenticate it through Zimbra.
Adding a Facebook Account to the Social Zimlet
To Add a Facebook Account to the Social Zimlet, click on Add/Remove Accounts at the top left. Click on the button to Add Facebook Account and follow the instructions to login to Facebook, and authenticate it through Zimbra. Read through the Permissions options carefully to ensure you select the option you prefer.
How to Socialize using the Social Zimlet
Once you have added a Twitter and/or Facebook account, you can quickly and easily access the Socialize button to add an entry to these accounts. From the Mail tab, you will see the Socialize button. Click on it and it will ask you 'What are you doing?'. Checkmark Facebook and/or Twitter depending on which social tool you wish to add the message to and then click on 'Update'.