Link prediction benchmark datasets
A cursory Google search gave me:


http://noesis.ikor.org/datasets/link-prediction
http://ial.eecs.ucf.edu/travian.php
http://konect.uni-koblenz.de/networks/


Another way would be to create your own dataset by downloading say Github user's graph, defining a network by adding edges  between user A and user B if A follows B and so on. Then you can take snapshots of this graph at different time instances and see if you can use last t snapshots to predict the edges in the t+1th snapshot. Of course that would be a lot more extra effort and the links I posted may suffice. Please let me know if it helps.