I guess its interesting to look at this question in terms of who and how they are trying to manipulate. Primarily are we talking about a committed group of people (i.e. a whole subreddit pushing a particular agenda across reddit as a whole) or an individual (perhaps this would be limited to less an agenda and more a certain "fact" or something? The large scale manipulation does happen online, and that's part of what brought the once hopeful site digg.com down. Or at least, after all that news about a large group of conservatives vote brigadng they seemed to fall off and now I never hear of anyone using it.
I think reddit is equally vulnerable here. Obviously vote brigading happens all the time between Feminists and MRAs, though this behavior is hardly particualr to hese groups or reddit as a whole. There have also been accusations of mod corruption and they've happened in various forms. I recall /r/skincareaddiction just finished cleaning house after it was found their mod team was quite systemic in their curating of posts, and more pointedly their effort to direct traffic to their own non-reddit affiliated site with all the usual monetization schemes.
Indeed, companies have long been aware of the value of having a valid online presence. I remember when Second Life was still something people cared about, businesses were actually setting up their own little kiosks in Second Life, and I think there may have even been some political campaigns which had a Second Life presence. PBS ran a documentary in the 90's called Merchants of Cool which touched on how companies will pay people to post on forums and chat rooms about their given products to enhance that brand's image.
The other end that I don't hear people talking about as much is that the Snowden leaks have revealed that the NSA has a very real program aimed at directing online dialogue. I'm not sure if this is limited to radical Muslim message boards or foreign websites, or if it includes domestic sites as well. Reddit of course has a few active subs like /r/sweden which operate in Swedish, so it is not unlikely that there could be a greater NSA presence here than other places like, The Chicago Sun-Times.
As for individuals, it seems like it would be a little less easy. I mean if you learn to play to the crowd you can certainly get more eyes on your message, and certainly I myself with go through upvoting things based on their titles alone while browsing the front page. There is a casual approach to information on Reddit because I think it is primarily used as a form of recreation, so concerns like verifying claims etc. probably take more of a backseat than they (hopefully) would to a news article.
That's just me spitballin' though.