1. A method comprising: receiving at a data processing system: (i) a first identifier of a user, U 1 , (ii) a first identifier of a tag, T 1 , and (iii) the number of times, M 1 , that tag T 1 has been associated with user U 1 ; and transmitting from the data processing system a second measure, M 2 , of a value of user U 1 , based on the value of M 1 .