Before you join a pile-on, ask if you’d still type it with the person sitting across from you.

Crowds make cruelty feel like participation. Inside a pile-on the target stops seeming like a person and becomes a subject, and the ordinary brakes that stop you saying something to a face do not engage against a username. The distance is the whole mechanism.

Run the check honestly before you type. Picture the person reading it while sitting opposite you, and see whether the sentence survives. If it does, say it, and if it does not, the crowd will manage without your contribution.