  Recent years have witnessed the explosion of online social networks (OSNs).
They provide powerful IT-innovations for online social activities such as
organizing contacts, publishing contents, and sharing interests between friends
who may never meet before. As more and more people become the active users of
online social networks, one may ponder questions such as: (1) Do OSNs indeed
improve our sociability? (2) To what extent can we expand our offline social
spectrum in OSNs? (3) Can we identify some interesting user behaviors in OSNs?
Our work in this paper just aims to answer these interesting questions. To this
end, we pay a revisit to the well-known Dunbar's number in online social
networks. Our main research contributions are as follows. First, to our best
knowledge, our work is the first one that systematically validates the
existence of the online Dunbar's number in the range of [200,300]. To reach
this, we combine using local-structure analysis and user-interaction analysis
for extensive real-world OSNs. Second, we divide OSNs users into two
categories: rational and aggressive, and find that rational users intend to
develop close and reciprocated relationships, whereas aggressive users have no
consistent behaviors. Third, we build a simple model to capture the constraints
of time and cognition that affect the evolution of online social networks.
Finally, we show the potential use of our findings in viral marketing and
privacy management in online social networks.
