If you go by what Zuck says, he calls this out in previous earnings reports and interviews[1]. It mainly boils down to 2 things:
1. Similar to other initiatives (mainly opencompute but also PyTorch, React etc), community improvements help them improve their own infra and helps attract talent.
2. Helping people create better content ultimately improves quality of content on their platforms (Both FoA & RL)
Sources:
[1]Interview with verge: https://www.theverge.com/23889057/mark-zuckerberg-meta-ai-el...
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> Zuck: ... And we believe that it’s generally positive to open-source a lot of our infrastructure for a few reasons. One is that we don’t have a cloud business, right? So it’s not like we’re selling access to the infrastructure, so giving it away is fine. And then, when we do give it away, we generally benefit from innovation from the ecosystem, and when other people adopt the stuff, it increases volume and drives down prices.
> Interviewer: Like PyTorch, for example?
> Zuck: When I was talking about driving down prices, I was thinking about stuff like Open Compute, where we open-sourced our server designs, and now the factories that are making those kinds of servers can generate way more of them because other companies like Amazon and others are ordering the same designs, that drives down the price for everyone, which is good.