  We present the first comprehensive characterization of the diffusion of ideas
on Twitter, studying more than 4000 topics that include both popular and less
popular topics. On a data set containing approximately 10 million users and a
comprehensive scraping of all the tweets posted by these users between June
2009 and August 2009 (approximately 200 million tweets), we perform a rigorous
temporal and spatial analysis, investigating the time-evolving properties of
the subgraphs formed by the users discussing each topic. We focus on two
different notions of the spatial: the network topology formed by
follower-following links on Twitter, and the geospatial location of the users.
We investigate the effect of initiators on the popularity of topics and find
that users with a high number of followers have a strong impact on popularity.
We deduce that topics become popular when disjoint clusters of users discussing
them begin to merge and form one giant component that grows to cover a
significant fraction of the network. Our geospatial analysis shows that highly
popular topics are those that cross regional boundaries aggressively.
