Sock puppet armies aren't science fiction:
[1]https://m.reddit.com/r/netsec/comments/38wl43/we_used_sock_p...
And chat bots are now being powered by NLP.
[2]http://www.theverge.com/2016/3/24/11297050/tay-microsoft-cha...
Subverting an online community would not be difficult for anyone with some programming knowledge and a desire to learn.
The study in the link[1] notes that detection is exceedingly difficult, and even then, in the case of Reddit, the Mods and Admins have negligible power to mitigate such attacks even if they do detect them.