Link creation in aNobii. Left: Distribution at snapshot 4 of the distances of nodes which become linked between snapshots 4 and 5, compared with the distribution at snapshot 4 of distances between all pairs of users. The points in give the portion of pairs of nodes between which no directed path exists in snapshot 4. Right: Measure of the preferential attachment. is the probability for a new node to create a link toward a node of degree k. Black circles and red squares account for in and out degree, respectively. Thick dots represent the log-binning of the values, and the dashed line represents a linear relationship .