People's influence has been the subject of study of several social and humanities disciplines. Lately, the study of user�s influence in micro-blogging platforms arises as an important issue. Although social influence or prestige can be defined as the potential or ability of an individual to engage others in a certain act, or to induce others to behave in a particular manner, there is no global consensus on what means to be an influential user. This work aims at shedding some light on how to assess user influence by proposing a novel metric of user influence based on analysing user behaviour regarding both content-based and topological factors. The metric does not only consider each user individually, but also aims at assessing the interactions with his/her neighbourhood. The statistical analysis performed confirmed that only analysing the topological factors is not sufficient for accurately assessing the influence of users. Instead the published content and its influence over the neighbourhood of users has to be also analysed. A comparison with a human assessment of user influence showed that the factors considered by the proposed metric are truly relevant for assessing people�s influence.