  The social brain hypothesis fixes to 150 the number of social relationships
we are able to maintain. Similar cognitive constraints emerge in several
aspects of our daily life, from our mobility up to the way we communicate, and
might even affect the way we consume information online. Indeed, despite the
unprecedented amount of information we can access online, our attention span
still remains limited. Furthermore, recent studies showed the tendency of users
to ignore dissenting information but to interact with information adhering to
their point of view. In this paper, we quantitatively analyze users' attention
economy in news consumption on social media by analyzing 14M users interacting
with 583 news outlets (pages) on Facebook over a time span of 6 years. In
particular, we explore how users distribute their activity across news pages
and topics. We find that, independently of their activity, users show the
tendency to follow a very limited number of pages. On the other hand, users
tend to interact with almost all the topics presented by their favored pages.
Finally, we introduce a taxonomy accounting for users behavior to distinguish
between patterns of selective exposure and interest. Our findings suggest that
segregation of users in echo chambers might be an emerging effect of users'
activity on social media and that selective exposure -- i.e. the tendency of
users to consume information interest coherent with their preferences -- could
be a major driver in their consumption patterns.
