Many of them in my opinion are jealous teens with too much time on their hands and nothing sensible to do.
I have been asked by friends has my work been trashed, the answer is no, for many reasons:-
A) I don't care what they say, my few readers know how well I can write, so they are not going to take notice of some mindless individual who has no idea what the books are about.
B) I refuse to rise to their barbs and as such defeat the object of the attack.
C) I am not connected to their main target who is a man in the USA, he makes a big deal of them on line, which only feeds their desire for attention - he should know better - however being a friend of his makes you a target.
The only work of mine which did get attacked is my charity work for http://alsdomain.weebly.com/the-peacock-writers.html#.Ud0vvTvVAW4 and this was not an attack on me but the book in general, because some of the writers are his friends.
Last year, my work on Goodreads was attacked by one such troll; I only learned about it from a friend of a friend as I never use Goodreads http://alsdomain.weebly.com/1/post/2013/04/i-must-be-good.html#.Ud0wVjvVAW4