Many businesses want to block facebook on their corporate networks. Here is a useful site to lookup blocks of IPs.
http://www.robtex.com/dns/www.facebook.com.html#records
http://bgp.he.net/AS32934#_prefixes
Using the IPs from above we can create firewall rules to block facebook.
/ip firewall filter add action=drop chain=forward comment="Block Facebook" dst-address=173.252.64.0/18 add action=drop chain=forward comment="Block Facebook" dst-address=204.15.20.0/22 add action=drop chain=forward comment="Block Facebook" dst-address=66.220.144.0/20 add action=drop chain=forward comment="Block Facebook" dst-address=31.13.64.0/18 add action=drop chain=forward comment="Block Facebook" dst-address=31.13.24.0/21 add action=drop chain=forward comment="Block Facebook" dst-address=69.171.224.0/19 add action=drop chain=forward comment="Block Facebook" dst-address=74.119.76.0/22 add action=drop chain=forward comment="Block Facebook" dst-address=103.4.96.0/22