  We study the diffusion of information in an overlaying social-physical
network. Specifically, we consider the following set-up: There is a physical
information network where information spreads amongst people through
conventional communication media (e.g., face-to-face communication, phone
calls), and conjoint to this physical network, there are online social networks
where information spreads via web sites such as Facebook, Twitter, FriendFeed,
YouTube, etc. We quantify the size and the critical threshold of information
epidemics in this conjoint social-physical network by assuming that information
diffuses according to the SIR epidemic model. One interesting finding is that
even if there is no percolation in the individual networks, percolation (i.e.,
information epidemics) can take place in the conjoint social-physical network.
We also show, both analytically and experimentally, that the fraction of
individuals who receive an item of information (started from an arbitrary node)
is significantly larger in the conjoint social-physical network case, as
compared to the case where the networks are disjoint. These findings reveal
that conjoining the physical network with online social networks can have a
dramatic impact on the speed and scale of information diffusion.
