As Ben Thompson pointed out recently, unlike Google and OpenAI, Meta benefits from open source AI taking off because that makes everyone better content creators, which accrues further value to their social media platforms.
EDIT: On reflection, you can probably extend the content creation argument to say that noncommercial tools enabling that without enabling commercial competition, to the extent that some of the models will be integrated into Meta products, is the best of all worlds for Meta, so the basic argument works even without open source in the strict sense.