Hi,

You can use application control to block all facebook and Youtube as applications regardless of how users access the website (http or https).

OR if you need to use webfilter feature to block streaming and social categories but for this to work properly you need to import fortigate ssl certificate into users machines or if you have a trusted CA in your domain or company then you will need to import this CA certificate into Fortigate appliance.

Keep in mind that youtube website can be accessed with https only

hope i answered your question