  An increasing fraction of today social interactions occur using online social
media as communication channels. Recent worldwide events, such as social
movements in Spain or revolts in the Middle East, highlight their capacity to
boost people coordination. Online networks display in general a rich internal
structure where users can choose among different types and intensity of
interactions. Despite of this, there are still open questions regarding the
social value of online interactions. For example, the existence of users with
millions of online friends sheds doubts on the relevance of these relations. In
this work, we focus on Twitter, one of the most popular online social networks,
and find that the network formed by the basic type of connections is organized
in groups. The activity of the users conforms to the landscape determined by
such groups. Furthermore, Twitter's distinction between different types of
interactions allows us to establish a parallelism between online and offline
social networks: personal interactions are more likely to occur on internal
links to the groups (the weakness of strong ties), events transmitting new
information go preferentially through links connecting different groups (the
strength of weak ties) or even more through links connecting to users belonging
to several groups that act as brokers (the strength of intermediary ties).

Physics and Society;Social and Information Networks