The platforms obviously bear major responsibility for this but to an extent their hands are tied by dispute about what constitutes "valid" speech.
My perspective is that if this had been an "organic" protest it would have been fine.
What TikTok demonstrated is that the platform is willing to directly spread misinformation to users through system messages with custom UX flows to magnify the impact. Devs at TikTok worked on this, they had meetings about it, they tracked progress until the work was done etc. That's a CONTROL PLANE issue.
Of course TikTok tried to get their userbase to protest Congress if Congress was trying to regulate them. That is entirely within their rights, and just about any American platform would have done the same, including distorting the message as much in their favor as possible. That's politics as it's meant to be played.
Can you name a prior instance where a social platform has added custom UX to "contact your congressman" or similar? (Bonus points if it is also about something actually untrue like "the service will be shut down").
[1]: https://en.wikipedia.org/wiki/Protests_against_SOPA_and_PIPA
I have some weak arguments for why I feel that case was different but I agree you refuted my point.