I am successfully sharing my Internet from my Macbook with others but I wonder is there a way I can block facebook.com or other sites for my users keeping my access to them?
Yes there is! Check out OpenDNS or just adjust local parental controls. OpenDNS will prevent your users from resolving the DNS for facebook.com. The only problem is you'll need to change their DNS servers yourself.
Alternatively, you can change it on your connection and then use your OpenDNS username and password to get through.