Comparing Facebook to an undirected graph and Twitter to a directional one sounds confusing. Nice article though!
Social networks are a pretty standard example of graphs (if you take a look at some CS curricula, you'll notice they come up again and again as widely-used examples).
Facebook or LinkedIn are both examples an undirected graph as "friendships" work two-ways. But on Twitter or Google+, the concept of "followers" rather than "friendships" make these graphs directed, and not bidirectional.
Hope this clears that up.
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