They participate because the Digital Markets Act is forcing them to: https://commission.europa.eu/strategy-and-policy/priorities-2019-2024/europe-fit-digital-age/digital-markets-act-ensuring-fair-and-open-digital-markets_en
Examples of the “do’s” - Gatekeeper platforms will have to:
- allow third parties to inter-operate with the gatekeeper’s own services in certain specific situations
- allow their business users to access the data that they generate in their use of the gatekeeper’s platform
- provide companies advertising on their platform with the tools and information necessary for advertisers and publishers to carry out their own independent verification of their advertisements hosted by the gatekeeper
- allow their business users to promote their offer and conclude contracts with their customers outside the gatekeeper’s platform
The interoperability is the big one. The Fediverse gives a way for Meta to be in compliance, and they have an interest in maintaining competition.