  In the online social dynamics, a robust scaling behaviour appears as a key
feature of many collaborative efforts that lead to the new social value. The
underlying empirical data thus offer a unique opportunity to study the origin
of self-organised criticality in social systems. In contrast to physical
systems in the laboratory, various human attributes of the actors play an
essential role in the process along with the contents (cognitive, emotional) of
the communicated artefacts. As a prototypal example, we consider the social
endeavour of knowledge creation via Questions\ \& Answers (Q\&A). Using a large
empirical dataset from one of such Q\&A sites and theoretical modelling, we
examine the temporal correlations at all scales and the role of cognitive
contents to the avalanches of the knowledge creation process. Our analysis
shows that the long-range correlations and the event clustering are primarily
determined by the universal social dynamics, providing the external driving of
the system by the arrival of new users. While the involved cognitive contents
(systematically annotated in the data and observed in the model) are crucial
for a fine structure of the developing knowledge networks, they only affect the
values of the scaling exponents and the geometry of large avalanches and shape
the multifractal spectrum. Furthermore, we find that the level of the activity
of the communities that share the knowledge correlates with the fluctuations of
the innovation rate, implying that the increase of innovation may serve as the
active principle of self-organisation. To identify relevant parameters and
unravel the role of the network evolution underlying the process in the social
system under consideration, we compare the social avalanches to the avalanche
sequences occurring in the field-driven physical model of disordered solids.
