Not sure where you got the idea that pfblocker would be the correct tool for blocking users from accessing social media, and allowing others.
That would be better done with a proxy and categories..
Sure you can use the pfblocker dnsbl to block domains... But there is no drop down list that says this social media site, that one, this one.. Allow X but block Y, etc.
These different sites use many many IPs multiple domain names and different CDNs to host their content.. Do you have specific block lists already in mind that your wanting to use with pfblocker?