Theyre gonna pay for it over the course of a decade. Meta revenue is still increasing 10% every year, a $50 billion lease payment on datacenters that are only worth $10 billion is annoying but they can handle it. Also this isnt actually debt, there is no interest. Its commitments theyve made to lease datacenters. A lot of the money here is a $10 billion dollar payment they owe a decade from now.

Meta will realistically live for decades, that's how they'll pay, don't forget that those companies got some of the best financial teams in the world, it's not even at a scale that regular people can comprehend.