To enable/disable SMS.Social, go to the menu in the top left. Expand "Account" and then select "Settings".
Once in your settings, you will notice the SMS.Social box. By default this will be set to "Organisational default".
To enable it, change the drop down to "enabled"
To disable it, change the drop down to disabled.
Once you are happy with what you have selected, click "save" at the bottom of the page.