  Internet users and businesses are increasingly using online social networks
(OSN) to drive audience traffic and increase their popularity. In order to
boost social presence, OSN users need to increase the visibility and reach of
their online profile, like - Facebook likes, Twitter followers, Instagram
comments and Yelp reviews. For example, an increase in Twitter followers not
only improves the audience reach of the user but also boosts the perceived
social reputation and popularity. This has led to a scope for an underground
market that provides followers, likes, comments, etc. via a network of
fraudulent and compromised accounts and various collusion techniques.
  In this paper, we landscape the underground markets that provide Twitter
followers by studying their basic building blocks - merchants, customers and
phony followers. We charecterize the services provided by merchants to
understand their operational structure and market hierarchy. Twitter
underground markets can operationalize using a premium monetary scheme or other
incentivized freemium schemes. We find out that freemium market has an
oligopoly structure with few merchants being the market leaders. We also show
that merchant popularity does not have any correlation with the quality of
service provided by the merchant to its customers. Our findings also shed light
on the characteristics and quality of market customers and the phony followers
provided. We draw comparison between legitimate users and phony followers, and
find out key identifiers to separate such users. With the help of these
differentiating features, we build a supervised learning model to predict
suspicious following behaviour with an accuracy of 89.2%.
