Unfortunately, unless you have a contract of some sort to the contrary, you can't refuse. An employer may make obtaining and having a social media account (e.g. Facebook) a condition or term of employment, just as an employer can require employees to be able to provide their own transportion to work, to have cell phones so they can be reached when offsite, to have acceptable clothing for work, etc. While it's understandably annoying, there is nothing illegal about an employer requiring employees to have and maintain their own Facebook or other accounts as a condition of employment; and if you don't honor this condition, you could be terminated (though if you're civil service, they'd have to go through the proper process and meet whatever your civil service requirements for  termination are).