  Selective exposure is the main driver for the economy of attention when
consuming online content. We select information adhering to our system of
beliefs and ignore dissenting information. However, even personal interest is
likely to play a role in determining our attention patterns. To understand in
more detail the dynamics of interest-driven choices, we address the evolution
of users' interest on Reddit by means of an analysis on more than 7 million of
users. We observe that the lifetime distribution of users on subreddits (online
thematic communities) is 'short' with respect to the observation period.
Furthermore, users tend to be active on a very limited number of subreddits
with respect to the wide offer of the platform. These aspects indicate the
presence of a migrating behavior of users among subreddits. To characterize
this phenomenon we propose a metric based on a geometrical encoding of the
'interest space' of the user. The movement of users across subreddits is
characterized by a bursty trend, made of sudden variations. The most frequent
of them take place between recreational subreddits and those more related to
news and politics. We describe this kind of activity with two behaviors called
interest drift and interest shift. Our results suggest that selective exposure
and personal interest coexist in driving content consumption.
