Are they the same thing?
Criticism can be harrasment, but harrasment does not nessecary be criticism.
I would imagine the line would be somewhere that concensus (sometimes in the form of the law) say it is. The target of the criticism alone would not be a good measure by itself (I could imagine large polluters would find protest against them harrasment a lot sooner than what the general public would think it is.).
Does the line then become visible when it targets individuals? I think it is a very interesting question.