The viral growth of online social networking applications in the
past few years has facilitated, like never before, forming new
friends online and keeping in touch with old friends and past
colleagues. These sites typically make it easy to declare friends, or
add like-minded people as friends, and then follow their activities
or posts online. While these declared networks appear large and
thriving, it has been recently shown [4] [10] that much of the
activity in these networks is driven by a more intimate group of
users. Twitter networks of friends and followers, for example, are
sustained [4] by an underlying sparse network of friends who
interact frequently and reciprocate each other’s attention. Even in
the social networks formed through mobile phone calls and text
messages--these overlap substantially with a user’s ‘real’ social
network [8]--an analysis of phone communication logs [10]
reveals that people interact with only a small fraction of the