You’re right, it isn’t entirely true. A perfectly private social media service could be constructed but it likely won’t because there is no profit motive. Even if it were, how many people would truly use it.
We had things like this in the beginning of the internet. Anonymous chat rooms and message boards were incredibly popular but I doubt they will ever reach that level of popularity again.
There is an actual section of the american constitution stating that a state cannot prosecute their citizen for something they legally did in another state