  Social networks readily transmit information, albeit with less than perfect
fidelity. We present a large-scale measurement of this imperfect information
copying mechanism by examining the dissemination and evolution of thousands of
memes, collectively replicated hundreds of millions of times in the online
social network Facebook. The information undergoes an evolutionary process that
exhibits several regularities. A meme's mutation rate characterizes the
population distribution of its variants, in accordance with the Yule process.
Variants further apart in the diffusion cascade have greater edit distance, as
would be expected in an iterative, imperfect replication process. Some text
sequences can confer a replicative advantage; these sequences are abundant and
transfer "laterally" between different memes. Subpopulations of the social
network can preferentially transmit a specific variant of a meme if the variant
matches their beliefs or culture. Understanding the mechanism driving change in
diffusing information has important implications for how we interpret and
harness the information that reaches us through our social networks.
