Use some parental control applications to block the Social networking sites in Server operating system.
Just keep in mind, these configs in IE to prevent users from accessing certain websites can all be worked-around. To truly block/filter website access requires a firewall/proxy solution. First, at the perimeter firewall, you block ALL outbound internet access from all hosts except from the proxy server. Then on the proxy server, you configure your rules and filters to control which users are allowed outbound access and to the sites/categories that are allowed. Microsoft's ISA/Forefront solution is not very expensive and does a very nice job. You can get some free proxy software out there such as SQUID, but it takes a bit of work to get the authentication portion configured properly. Alternatively, you can opt for using hte proxy and simply deploy a firewall that has URL filtering capabilities. URL filtering is the most basic where you would input the URLs that are allowed/disallowed for outbound internet access.