A directed graph of the network of questions-and-responses on Stack
Exchange.  The graph is based on the March 2013 data dump of Stack
Exchange and downloaded from
http://www.clearbits.net/creators/146-stack-exchange-data-dump.  The
file stackexchange.edge contains the edge list of the directed graph.
Each line represents an edge of the form "u v", which denotes that
user with ID u responded to a question or comment of the user having
ID v.

Nodes:   852,333
Edges: 8,442,853
