Facebook is planning to join the Fediverse by releasing a Twitter-like service that runs on ActivityPub. This would flood the Fediverse with content (and trolls) from Facebook, allow Facebook to mine data from the Fediverse, and would do irreparable damage to this small part of the internet which has not yet been marred by corporate greed.
Will the admins of this instance agree to block any instance that is set up by Facebook?
Defederation cuts off the connection between two servers both ways. In this case, it would actually prevent facebook from harvesting data from these servers, and the server would not get any of the facebook server traffic.
That all being said, the objection to facebook joining the fediverse is partly a moral one, and a fear that facebook will try to EEE the fediverse (as in, embrace, extend, extinguish) like they did with other protocols.
There are obviously lots of reasons to be skeptical of meta, and to consider not federating with them, but
isn’t the case. They can already mine data it if they really want – lots of folk set up bots to scrape mastodon data.