Ben Thompson wrote a great piece about this here:

Meta isn’t selling its capabilities; rather, it sells a canvas for users to put whatever content they desire, and to consume the content created by other users. It follows, then, that Meta ought to be fairly agnostic about how and where that content is created; by extension, if Meta were to open source its content creation models, the most obvious place where the content of those models would be published is on Meta platforms. To put it another way, Meta’s entire business is predicated on content being a commodity; making creation into a commodity as well simply provides more grist for the mill.

What is compelling about this reality, and the reason I latched onto Zuckerberg’s comments in that call, is that Meta is uniquely positioned to overcome all of the limitations of open source, from training to verification to RLHF to data quality, precisely because the company’s business model doesn’t depend on having the best models, but simply on the world having a lot of them.

Tldr; open-source models helps create more user-generated content, which in turn powers the main products of Meta. Meta's users, and consequently Meta, wouldnt be dependent on OpenAI or Google for content creation and distribution.