  We investigate two paradigms for studying the evolution of
cooperation--Prisoner's Dilemma and Snowdrift game in an online friendship
network obtained from a social networking site. We demonstrate that such social
network has small-world property and degree distribution has a power-law tail.
Besides, it has hierarchical organizations and exhibits disassortative mixing
pattern. We study the evolutionary version of the two types of games on it. It
is found that enhancement and sustainment of cooperative behaviors are
attributable to the underlying network topological organization. It is also
shown that cooperators can survive when confronted with the invasion of
defectors throughout the entire ranges of parameters of both games. The
evolution of cooperation on empirical networks is influenced by various network
effects in a combined manner, compared with that on model networks. Our results
can help understand the cooperative behaviors in human groups and society.
