How can I block social media sites on a network using windows server 2012 r2?

you can use Pfsense this is Firewall software,it's free and very easy to use

regards

you can use TMG with win server 2012 R2

On your network, yes you can block social media sites by using firewall configurations, but on your server, there is no way this can be done. 

most simpe way create a script using gpo to edit clients host file n that script add which sites you need to block. then push using gpo