Best way to block such site is using Router Setting if possible.
Prental control and Content Advisors are also Options but Are some traditional and annoying to Employee.
Best method to do so is Change some setting in Hosts File. I am going to tell you about that. To do so, You have to change some setting in individual PCs for doing so.
Open Notepad with Administrator Rights.
Ctrl+O to Open File. Choose File Path Below.
Windir:(C: )\Windows\System32\drivers\etc
Change File Type to All Files from Right Bottom Corner.
Choose hosts file and open it.
You Will See below Lines at Last of Page
# 127.0.0.1 localhost
# ::1 localhost
Now Just add this code to Block facebook
Using IPv4
127.0.0.1 www.facebook.com
127.0.0.1 facebook.com
Using IPv6
::1 www.facebook.com
::1 facebook.com
Now Just save this File. If you are using Antivirus, This may disinfect it, So Disable Real Time Protection for some time and After saving changes enable those Setting of Antivirus.