Defederating from Facebook just seems like it's motivated by giving them the middle finger.

Well I've found where we disagree! I think defederating from Threads is more of a safeguard of the users of this instance from Meta's predatory data practices, and history of employing the classic Embrace, Extend, Extinguish strategy. Personally, I think having zero connection to anything run by Meta is certainly giving the users of the instance the best possible experience.

I think considering how incredibly harmful Facebook/Meta has been to society, and that Meta intentionally pushes divisive and hateful content because it creates the most engagement, it makes sense from a pragmatic point of view to simply pre-emptively block them in all ways possible. Their entire history as a company has shown they will make anyone who uses their services, and society as a whole, worse off. Intentional or otherwise. It need not be motivated from a metaphorical middle finger, but as self-defense against a practical certainly of something bad coming from interaction with them.

It takes only a short glance at the 'Facebook papers' leak to see why it makes sense to distance ourselves as much as possible:

The so-called “Facebook Papers” include a mix of presentations, research studies, discussion threads and strategy memos. What the documents reveal about Facebook’s behavior is stark and damning. They show how some of Zuckerberg’s public claims about Facebook’s principles and activities clashed with internal company findings. For example, he once told Congress that Facebook removes 94 percent of the hate speech it finds. But the inverse was true — according to internal estimates, the number was probably less than 5 percent.

Ahead of the Jan. 6 assault on the Capitol, Facebook’s efforts to stem the flow of misinformation proliferating on its networks fell short. Company employees were unhappy as far-right groups spread the call to join the “Stop the Steal” rally that preceded the attack.

“This is not a new problem,” one unnamed employee fumed on Workplace, an internal message system, on Jan. 6. “We have been watching this behavior from politicians like Trump, and the — at best — wishy washy actions of company leadership, for years now. We have been reading the [farewell] posts from trusted, experienced and loved colleagues who write that they simply cannot conscience working for a company that does not do more to mitigate the negative effects on its platform.”

The Facebook Papers also make clear how Zuckerberg prioritized maximum engagement and the company’s bottom line over ethical concerns about safety and best practices. While he espouses a form of free speech maximalism in public in the United States, he has participated in enabling regimes of censorship elsewhere. My colleagues also pointed to a 2019 episode in Vietnam, where Zuckerberg personally decided to comply with demands from the autocratic government in Hanoi to censor dissident voices on his platform.

“Ahead of Vietnam’s party congress in January, Facebook significantly increased censorship of ‘anti-state’ posts, giving the government near-total control over the platform, according to local activists and free speech advocates,” my colleagues reported.

The Facebook Papers “are astonishing for two reasons,” wrote the Atlantic’s Adrienne LaFrance. “First, because their sheer volume is unbelievable. And second, because these documents leave little room for doubt about Facebook’s crucial role in advancing the cause of authoritarianism in America and around the world. Authoritarianism predates the rise of Facebook, of course. But Facebook makes it much easier for authoritarians to win.”

To assume that some good could come from interacting with Meta is, in my honest opinion, a naive stance. But that's just my two cents.

if the goal is to cast a judgement on their ethics

What if the goal is to use knowledge of their past behaviour to make an informed guess about their likely future behaviour?