Add a Web Access Layer. Name the object, for example, BlockFacebookChat.
Create new URLs in the list. Right click Destination and click Set. Select New > Request URL > Simple Match , and add these three URLs: facebook.com/ajax/chat, facebook.com/ajax/messaging, facebook.com/ajax/mercury Then close the Add Request URL Object page. Note: Facebook has combined chat and messaging functions.
Create a new rule within that newly created layer. Right-click Destination and click Set. Select New > Combined Destination Object. Give it a name such as FacebookChatURLs, and add those newly created three URLs into the same box on the right; leave the other box empty.
Back to the rule: Source = any, Service = any, Time = any, Action = Deny, Track = none.