This is an over–simplification of the facts. The Facebook website (or app, if you prefer) is still called Facebook. It's the parent company (which also owns Instagram and WhatsApp, among other such) that's changed its name, from Facebook Inc., to Meta Platforms Inc.
The idea behind the name change, reported in October 2021, is to "reflect [the company's] focus on building the metaverse". No one seems very sure what "the metaverse" actually is – unless it's another word for the multiverse ("the set of all universes") – but Wikipedia defines "a metaverse" variously as "the sum of all virtual spaces", "a network of 3D virtual worlds focused on social connection", and "a hypothetical iteration of the Internet as a single, universal virtual world that is facilitated by the use of virtual and augmented reality headsets."
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