It is difficult to work effectively when you have an associate who is effectively working against you. It is rather surprising that your manager has done nothing to stop this online bashing, as it is clearly divisive and disruptive. Besides, who is this co-worker who somehow thinks he is supposed to be criticizing your work?
Perhaps your manager has been encouraging him to play this role, and that is not a good sign. One tip-off is that if such tattling and prattling were not expected or accepted, your boss would have put a stop to it.
The best way to find out what is going on here is to meet with this co-worker and discuss the matter. You should try to use a question and answer approach and basically ask him why he is doing this. His answer is going to help you determine your next step. For example, if he says that the boss needs to know about these kinds of matters, then perhaps you should follow that guideline as well. If your co-worker thinks that you will be copying the boss on your communications to him, perhaps he will think twice about this whole matter.
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