  Cities are characterized by concentrating population, economic activity and
services. However, not all cities are equal and a natural hierarchy at local,
regional or global scales spontaneously emerges. In this work, we introduce a
method to quantify city influence using geolocated tweets to characterize human
mobility. Rome and Paris appear consistently as the cities attracting most
diverse visitors. The ratio between locals and non-local visitors turns out to
be fundamental for a city to truly be global. Focusing only on urban residents'
mobility flows, a city to city network can be constructed. This network allows
us to analyze centrality measures at different scales. New York and London play
a predominant role at the global scale, while urban rankings suffer substantial
changes if the focus is set at a regional level.
