yes sorry you have specified in title now that i see. it isn't that easy task you have to be familiar with Layer7 Protocol, Filter rules, Mangle, Queue Types.
the right direction to this is to:
1. create a Layer 7 protocol to detect facebook.com traffic regexp:
2. create a filter rule that will add dst address of detected traffic to an address list. use the Layer 7 entry created in step 1
3. create rules in mangle to mark connections and traffic to these dst address list
4. create a queue type pcq download of 5M rate
5. use the traffic mark and the queue type in a simple queue or queue tree
try following these steps yourself or tell here wich one you don't understand or if you want more detailed tutorial it needs a little more time to compile it