It is a bad thing, because the law protects our disagreement with the election deniers/MAGA diehards only as much as it protects their disagreement with ours. Having the same opinions as people who did illegal things isn't and shouldn't be grounds for targeted surveillance.
The pendulum will swing the other way eventually, and they'll be digging into the messages of people that talk about defunding the police because "antifa" took control over part of Portland for a while. Or target abortion activists because they caught a few people trying to assassinate Supreme Court justices.
Trusting that government overreach will only affect the other side usually backfires.
[Citation Needed]
I'll assume you mean the first amendment and that only applies to the government not to private business (i.e. Facebook). It definitely does not apply to a webpage that told you in their T&C that they're scanning the content you post.
The article may quote “Facebook provides the FBI with private conversations which are protected by the First Amendment without any subpoena.” but it wouldn't be worth the ink it's printed on. Time and time again it's been shown that if a third party (i.e. facebook) wants to willingly give information to the government a subpoena is not needed.
You're right that it's legal under the current interpretation of the laws. It just means the 4th Amendment isn't worth the paper it was printed on, and it'll get relegated to the "stuff people used to care about category" like the 3rd.
There's a huge difference between a cop asking a single person for their ID, and dragnet surveillance of people with a particular viewpoint.