  Twitter may be considered as a decentralized social information processing
platform whose users constantly receive their followees' information feeds,
which they may in turn dispatch to their followers. This decentralization is
not devoid of hierarchy and heterogeneity, both in terms of activity and
attention. In particular, we appraise the distribution of attention at the
collective and individual level, which exhibits the existence of attentional
constraints and focus effects. We observe that most users usually concentrate
their attention on a limited core of peers and topics, and discuss the
relationship between interactional and informational attention processes -- all
of which, we suggest, may be useful to refine influence models by enabling the
consideration of differential attention likelihood depending on users, their
activity levels and peers' positions.
