  In a diversified context with multiple social networking sites, heterogeneous
activity patterns and different user-user relations, the concept of
"information cascade" is all but univocal. Despite the fact that such
information cascades can be defined in different ways, it is important to check
whether some of the observed patterns are common to diverse contagion processes
that take place on modern social media. Here, we explore one type of
information cascades, namely, those that are time-constrained, related to two
kinds of socially-rooted topics on Twitter. Specifically, we show that in both
cases cascades sizes distribute following a fat tailed distribution and that
whether or not a cascade reaches system-wide proportions is mainly given by the
presence of so-called hidden influentials. These latter nodes are not the hubs,
which on the contrary, often act as firewalls for information spreading. Our
results are important for a better understanding of the dynamics of complex
contagion and, from a practical side, for the identification of efficient
spreaders in viral phenomena.
