“…Since Discord’s Terms of Service only allows people aged 13 or older to use their platform and Roblox is a platform for all ages, we cannot risk unfairly exposing underage users to platforms such as Discord…”
So we’re allowing Facebook to break that rule and link to kids social media private lives but something like Discord isn’t allowed? 
"…Facebook can be viewed without logging in "
With little or no real value except for seeing a meme video that may have been posted publicly or one of those videos that start off fine then scream NSFW stuff at you. I’ve seen some people link Facebook pages but that got removed for some reason, so only Facebook profiles can be linked now.