1. now Facebook can start designing against this. What design features promote negativity? Where is negativity being inadvertently fostered? 2. now everyone else can benefit from the research as they chose not to keep it private despite the potential PR ramifications like this 3. this is not research more than a few entities in the world can or will do.
Ethically, I don't see the problem. Users already rely on Facebook to tinker with user feeds in a variety of ways, and this tinkering causes them no harm, and may benefit them and all future users. (When the users signed up for Facebook, did they sign an agreement saying 'you agree to be mentally poisoned with rage, frustration, anger and disappointment, without recourse, by anyone you are foolish enough to friend and let flood your feed'? I rather doubt it.) Given that users are already buffeted by countless unpredictable random forces beyond their control or comprehension, what is tweaking one factor going to amount to? This is the same as clinical drug trials: the question should not be whether it is moral to run a trial, the question should always be whether it is moral to not run a trial.
The OP shows no sign of understanding any of this.