But then every firm that created a new product or new product category would be immediately illegal. That's why monopolies aren't illegal, not even in the EU.

But then every firm that created a new product or new product category would be immediately illegal. That's why monopolies aren't illegal, not even in the EU. They have to engage in abuse to create or maintain a monopoly. But Facebook hasn't done that. Buying your competitors is not abuse of market position, they willingly joined forces.