1 Introduction
2 A brief description of the Twitter network
2.1 Twitter metrics
3 Twitter API
4 What is an influential user?
5 Activity measures
6 Popularity measures
7 Influence measures
7.1 Traditional measures used on Twitter
7.2 Measures based on Twitter metrics and PageRank
7.3 Topical influential users
8 Some additional aspects
8.1 About computational complexity
8.2 About correlation of measures
8.3 Some experiments
9 Open research problems
10 Conclusions