In case you want to block access to Facebook, Twitter, Youtube or other websites, it is easy to do this job on Mikrotik RouterOS. You can use either web proxy or directly from firewall rule to block websites.

According to Mikrotik Wiki, you can block users from accessing websites using content option in Mikrotik Firewall rule. One rule per content you want to block. So if you want to block, for example, Facebook, Youtube, and Twitter, you have to create rule for each content.