Influencers (individuals or groups) over a selected audience (observers or recipients of information, objects and/or events) on a given topic are measured based on influence features, which include a sentiment flipping influence feature indicative of ability of an audience member to influence other audience members to change their sentiment on the selected topic. Other influence features include the ability to influence others: to change followership; to express interest in a topic associated with a hashtag pioneered by the influencer, based on the effectiveness and phrasing of language used. The output of the influence engine can be a score representing the relative influence of audience members over the audience on the topic of interest. Influencers may be ranked according to their total influence score over the audience on the topic.