They can already access the data, it's all federated and it's all publically available effectively by definition, they don't need to launch a platform that interacts with Fedi in order to scrape it. And Meta will only be able to scrape user profiling data on the people accessing Fediverse through their own tools and platforms. In the large term, all data is useful and getting the additional facets of how their users interact with a twitter-like platform is good - but I don't think that's really why they chose to federate.

But...

What joining Fediverse does offer them is a way of launching their Twitter-rival product with genuine and organic content or activity already present.

Facebook & Instagram's primary demographics are not internet pioneers, they don't tend to build new things - they feed off existing activity and build on top of it. They access the platforms to consume content, and only move to creating or posting content over time as they develop networks on the sites. Meta cannot realistically launch a Twitter competitor whole-cloth. The sort of people who joined Twitter early to build that space aren't joining a Meta product, likewise the people who join new platforms or normal fediverse.

If it launched empty, it would remain empty. People would check it out, see almost no content or no content they care about, and not come back. Meta can only realistically launch a product like Threads with activity already occurring, and things like AI content or fake profiles aren't necessarily convincing enough to lure in the punters. But Fedi is preexisting and active and there's already A Thing there that Meta can point their users at, there's already content to consume and people to interact with.

My opinion is it’s just the data, Meta is all about collecting data. Being part of the fediverse means they potentially get access to a bunch of data through scraping and user interactions.

You're absolutely right! Just take a look at the privacy of the Threads app. Even if you don't have Threads app installed, they can figure out anyone from another fediverse server. That's the reason fediverse servers should not federate with Meta servers.

Being part of the fediverse means they potentially get access to a bunch of data through scraping and user interactions.

They can already scrape this data without even being part of a federated community though.

That’s a very good point. There’s always an agenda when it comes to these massive companies. The more data they have, the more predictions they can make, and the more accurate they’ll be. Understanding how people move through these communities is massive for Meta. The more you understand something, the better you know how to exploit it.

They can scrape that data withiut federating. Anyone can, there's a public API. And I suspect once they federate Threads, Mastodon will be a tiny share of the resulting fediverse.

No to say they won't ingest all that data, of course, they'll get their hands on everything they can, but I doubt it's the primary motivation behind it.