The bigger the size of a network, the more attractive it is to abusers, while at the same time being more difficult to manage.
Typically, they are constrained by some limitation like being for an offline group or requiring a vouching process to join.
Importantly, a blockchain is not very fit for an application like this because it is append-only. That makes perfect sense for a transaction ledger, where you want immutable history and transactions are reversed by entering a new transaction with the signs reversed, but you really want the ability to correct a reputation history by actually redacting false reports rather than just appending a correction.
George Orwell's 1984 features a main character whose job is just this: rewriting history by 'redacting false reports'.
Of course well-meaning people will set up these systems without considering the potential for abuse. In many cases, persistence is desired - it does matter who said what when, historical written letters are important documents, and records of actions also form institutional histories. Normalizing the 'redacting of false reports' can easily turn into rewriting history for propaganda reasons.
I suppose you could have a two-lane social media system, one channel where comments and posts were not anonymous and were recorderd for posterity, and one channel for anonymous ephemeral chatter. However, trying to run both channels on one platform might not work, legally or technologically.