Improve all pairs distance (unweighted graph)
Could you optimize topology.shortest_distance in the case of an (undirected) unweighted graph? It seems that you use a general APSP algorithm with weight 1. In the calculation of the closeness centrality you've already optimized this by running bfs for all vertices.
See also, http://math.stackexchange.com/questions/58198/all-pairs-shortest-path-in-undirected-and-unweighted-graphs