Misusing "troll" to refer to online harassment (which is already vague) is annoying. Trolls do it "for the lulz". It is funny, to the troll, to see people overreact to things. The measure of trolling is the ratio of the troll's effort expended to the effort expended by the trolled, lower is better. Trolling is not good for forum quality, but it is not harmful to individuals. It isn't harassment, any more than rudeness is. You could harass someone by repeatedly trolling them, just like you could harass someone by repeatedly being rude to them, but they're still distinct. There are a few very online people that are notorious for being unable to resist the urge to respond to trolls.
What this article appears to be referring to is cyberbullying, not trolling. Cyberbullying is real and pervasive, especially among teenagers. It is more prevalent on social websites with private messaging. I've never seen it on HN. Trolling here is not unheard of.