There is a workaround to this. For some reason the Facebook contacts are categorised SNS (Samsung's Social Networking Service) and can't have phone numbers added. So what you need to do is create a new contact with the same name. Add the number and any other details to that and then use menu-Join contact to merge the two contacts. It's a bit long winded but it works. I read somewhere that a future version of Contacts for the Note (maybe the ICS version coming soon, hopefully) will solve this problem.