  The global public sphere has changed dramatically over the past decades: a
significant part of public discourse now takes place on algorithmically driven
platforms owned by a handful of private companies. Despite its growing
importance, there is scant large-scale academic research on the long-term
evolution of user behaviour on these platforms, because the data are often
proprietary to the platforms. Here, we evaluate the individual behaviour of
600,000 Twitter users between 2012 and 2019 and find empirical evidence for an
acceleration of the way Twitter is used on an individual level. This manifests
itself in the fact that cohorts of Twitter users behave differently depending
on when they joined the platform. Behaviour within a cohort is relatively
consistent over time and characterised by strong internal interactions, but
over time behaviour from cohort to cohort shifts towards increased activity.
Specifically, we measure this in terms of more tweets per user over time,
denser interactions with others via retweets, and shorter content horizons,
expressed as an individual's decaying autocorrelation of topics over time. Our
observations are explained by a growing proportion of active users who not only
tweet more actively but also elicit more retweets. These behaviours suggest a
collective contribution to an increased flow of information through each
cohort's news feed -- an increase that potentially depletes available
collective attention over time. Our findings complement recent, empirical work
on social acceleration, which has been largely agnostic about individual user
activity.
