Imagine that something fundamental shifts in the world of AI research. It could be anything: AI suddenly makes programmers much more productive, AI becomes very good at identifying vulnerabilities, AI chat becomes a new major source of entertainment, AI images become an item popularly shared on Instagram (etc)
Suppose any one of these things happened and suddenly Facebook wished that it had access to state of the art models so that it could customize them for its uses (internal developers or tools, embedding in their app).
Imagine how they would feel if the only way they could access these models were by signing 7-9 figure deals with a model dealer like OpenAI. Even worse, imagine if one of their main competitors in advertising started providing robust AI tools to help advertisers adapt their creatives to various form factors. Facebook is now way behind and possibly has to shell out millions to a company like OpenAI all while also losing ad market share worth billions per quarter (ads on Google start performing much better, so Google gets more ad spend)
If this worst case scenario came to pass, Facebook would look foolish. If even one of these things were likely their investments make sense. The rest (open source, make meta a cool place to work) are a strategy credit.