SEC, FTC, FCC all have regulatory authority over these publicly-traded corporations (not, repeat not, private companies).
CDA (1996) Section 230 specifically provides them with Federal protection from content liability in exchange for operating as “open platforms” that do not act as content publishers (editors).
We do not need to pass new legislation. We do not even need to invoke anti-trust regulations.
We need our Federal government to apply the laws on the books.
These corporations have engaged in fiscal malfeasance by causing politically-motivated scandals that have damaged stock value.
They have committed consumer fraud by applying their terms of service - a contract - in a selective and deceptive manner. (Users are consumers: Their data provides the platform with revenue, directly and/or indirectly.)
Finally, and most pertinently, they have manifestly violated the conditions for their legal immunity by acting as content publishers.
I've thought that for a long time. Their actions are a breach of their fiduciary responsibilities.