Every site which uses the Reddit upvoting model is subject to this type of attack. The correct name for it is a Sybil Attack[1]

I suspect a large portion of Reddit accounts are sockpuppet[2] accounts, alongside Product Hunt.

It's the Law of Manipulatable Numbers where if you put a number next to somebody's name online, then the person sometimes (not often) tries to manipulate the number.

[1]: https://en.wikipedia.org/wiki/Sybil_attack

[2]: https://en.wikipedia.org/wiki/Sockpuppet_(Internet)

Also noteworthy:

http://www.dailydot.com/layer8/trump-clinton-debate-online-p...