The arguments I've heard about tracking etc are misguided and don't understand the actual risks.
Firstly, posts on the fediverse are already likely being consumed by advertising platforms like Facebook & Google. It would be trivial for big tech companies to setup relays that act as scrapers.
Secondly, the value in platform's tracking individuals is for advertising. There is no mechanism for these platforms to identify you browsing the we if your instance federated with threads. Your instance won't share cookie sessions etc with threads. It doesn't increase your exposure.
Thirdly, these platforms have the know how to deal with spam and they will be incentivised to share that tech with other federated instances.
Don't get me wrong, Facebook is an evil company. But I haven't heard a decent argument as to why them joining the fediverse is a bad thing. We always have the option to defederate in the future.
Change my mind.
I'm aware of the EEE argument. It's just not valid in this instance. Let's follow the EEE argument. Facebook extends the capabilities of activitypub and makes third party users no longer compatible. Then what? We're in the exact same place we are now, Facebook having a wall garden and the rest of the fediverse doing its own thing.
You arw too optimistic, there is always a way, and it will probably not replace lemmy or kbin but maybe mastodon or any other healthy ActivityHub service. The danger is there, just because you don't know the answer to the the real danger doesn't mean there is no danger.
Facebook (or rather "Meta") has a buisness model, and its there to damage us all, grow into infinity and be the number one. They will become creative and always take an unhealthy turn whenever its possible.
With very little probability it may become an utopia where everything will be nicely federated and still Open and Open Source. With high probability they will focus on full destruction to gain the most money out of any situation.