If you're being genuine, the answer is:
* They allowed for advertising campaigns using their tools which were flat out illegal (e.g.: Real-estate advertising targeted based on race.)
* They have failed to meaningfully control over illegal uses of their platform in any way. (See: The current Rohingya issue)
* They were far too liberal with access to user data, in a way that is currently driving a debate around the legality of doing so. (Cambridge Analytica, etc.)
* They have built tools that are ideal for doing things like illegally swaying elections. ("Russia doing stuff".)
I can only assume there are many other things that Facebook have made possible which are illegal and completely unnoticed. It's hard to try to build any sort of exhausted list of unknown unknowns, though, so I'll leave it at a prominent four.
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If you're using rhetoric, the thing you'll probably say next is "but that's not Facebook being bad, that's people using Facebook! You can't blame a tool for how it's used!", but the problem with that is that Facebook exerted editorial control over the platform in numerous ways over the time period in which these things happened, both in advertising and in news-feed sorts of ways.
Any person doing any of these things would be punished.
Any person aiding and abetting any person doing any of these things would be punished.
Why not Facebook?