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Having the great majority of the customers IS a monopoly. And also trying to buy the rest is (e.g. instagram, whatsapp).

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.