Example of diffusion tree for a photo p uploaded into the photo pool of group g . Circles represent users and the dashed line marks the boundaries of the group. Red circles are the root users. In this example 8 users out of 20 members are nodes of the diffusion tree, leading to . Also, 3 users outside the group are nodes of the tree, for a total tree size of 11 nodes (except the meta-root), thus leading to .