Calling it "harassment" is perhaps a bit misleading if it's people gossiping about and making fun of someone behind their back. Harassment generally involves intentional (as opposed to incidental) communication to the target. The issue is that this forum (like Twitter and other forums) is generally readable by the public, so someone can observe two people saying stuff about them, but it's not addressed to them.

It might be more like stalking, but one could also argue that writing a hit piece in a mainstream publication is also stalking as it could intimidate the target.