How does anti-trust legislation work for these kind of sites? If Facebook was split up then what would stop a single identical entity from popping up in some ot

How does anti-trust legislation work for these kind of sites? If Facebook was split up then what would stop a single identical entity from popping up in some other country that fills the niche for the entire world? I've seen it mentioned in various places on this site but I can't understand how it would work in practice.