document: We know who you followed last summer : inferring social link creation times in twitter Understanding a network 's temporal evolution appears to require multiple observations of the graph over time . These often expensive repeated crawls are only able to answer questions about what happened from observation to observation , and not what happened before or between network snapshots . Contrary to this picture , we propose a method for Twitter 's social network that takes a single static snapshot of network edges and user account creation times to accurately infer when these edges were formed . This method can be exact in theory , and we demonstrate empirically for a large subset of Twitter relationships that it is accurate to within a few hours in practice . We study users who have a very large number of edges or who are recommended by Twitter . We examine the graph formed by these nearly 1,800 Twitter celebrities and their 862 million edges in detail , showing that a single static snapshot can give novel insights about Twitter 's evolution . We conclude from this analysis that real-world events and changes to Twitter 's interface for recommending users strongly influence network growth .
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