Who says algorithms are the only possible determinants of something which is trending? The article specifically points out that human editors were introduced a

Who says algorithms are the only possible determinants of something which is trending? The article specifically points out that human editors were introduced after Facebook failed to surface Ferguson protests—making it look bad because those protests were in fact trending on Twitter. Moreover, some theoretical perfect algorithm could have asserted that was a trending topic. In fact, there is not perfect algorithm for determining what is "trending." Part of hiring human curators is to train an algorithm which can more effectively use judgement in figuring out topics which are truly trending/newsworthy/interesting instead of coincidentally used a lot without being valuable (ex. #FollowFriday). Moreover, it seems clear that injecting stories is actually relatively rare. The primary reason to have human curators is to do filtering to prevent the junk from showing up in trending.