Most likely, a number of nxb's friends have Facebook integrated with the contacts list on their smartphone, and have nxb's birthday associated with their contac

Most likely, a number of nxb's friends have Facebook integrated with the contacts list on their smartphone, and have nxb's birthday associated with their contact details there. Another option is if nxb's Facebook account has been connected to their Skype account (though from their FB-skeptic stance I find it unlikely they would make such a connection in the first place).