  Most studies on social influence have focused on direct influence, while
another interesting question can be raised as whether indirect influence exists
between two users who're not directly connected in the network and what affects
such influence. In addition, the theory of \emph{complex contagion} tells us
that more spreaders will enhance the indirect influence between two users. Our
observation of intensity of indirect influence, propagated by $n$ parallel
spreaders and quantified by retweeting probability on Twitter, shows that
complex contagion is validated globally but is violated locally. In other
words, the retweeting probability increases non-monotonically with some local
drops.
