  Recent findings showed that users on Facebook tend to select information that
adhere to their system of beliefs and to form polarized groups -- i.e., echo
chambers. Such a tendency dominates information cascades and might affect
public debates on social relevant issues. In this work we explore the
structural evolution of communities of interest by accounting for users
emotions and engagement. Focusing on the Facebook pages reporting on scientific
and conspiracy content, we characterize the evolution of the size of the two
communities by fitting daily resolution data with three growth models -- i.e.
the Gompertz model, the Logistic model, and the Log-logistic model. Then, we
explore the interplay between emotional state and engagement of users in the
group dynamics. Our findings show that communities' emotional behavior is
affected by the users' involvement inside the echo chamber. Indeed, to an
higher involvement corresponds a more negative approach. Moreover, we observe
that, on average, more active users show a faster shift towards the negativity
than less active ones.
