I could definitely see a benefit for them from a legislative perspective. By federating with other networks, they're able to signal to lawmakers that they're not _really_ a monopoly, they're willing to pay-ball with others.

Also, isn't the 100M user figure disputed, because it's counting existing Instagram users or some such?

It is real sign ups but it is very easy to sign up. Just because you signed up doesn't mean you're going to use it regularly.

The EU digital markets act will require "gatekeepers" like meta to provide some form of interoperability or open access. Supporting activitypub would be a way to satisfy that requirement.

But Mastodon can't possibly comply with GDPR the way it is organized. I mean Heavens you can use it without clicking on a cookie popup, they probably owe $70 billion dollars just for all the people who haven't seen a cookie popup already.

I'm assuming this is the point. Facebook can get around things like requests for deletion under the GDPR by sending that data out to the fediverse, and then reading it back in from the fediverse after they've deleted it.

and when the EU complains, they get to throw their hands in the air and say "yeah, you made us do it"

It doesn’t need a cookie popup because they aren’t using cookies for non-essential reasons, similar to how it complies with GDPR because they aren’t collecting any data beyond what is necessary for the service’s stated purpose.