How?

I would have said a flood of LLM-generated spam is a pretty big threat to Facebook's business. Facebook don't seem to have any shortage of low/medium quality content; it's not like they need open-weights LLMs to increase the supply of listicles and quizzes, which are already plentiful. There isn't much of a metaverse angle either. And they risk regulatory scrutiny, because everyone else is lobotomising their models.

And if they wanted a flood of genai content - wouldn't they also want to release an image generation model, to ensure instagram gets the same 'benefits' ?

Sure there are some benefits to the open weights LLM approach that make them better at LLMs - I'm sure it makes it easier for them to hire people with LLM experience for example - but that's only helpful to the extent that Facebook needs LLMs. And maybe they'll manage to divert some of that talent to ad targeting or moderation - but that hardly seems 'perfectly aligned', more of a possible indirect benefit.