An undirected graph is equivalent to a directed graph with doubled edges.
The only efficient way to implement an undirected graph is as a directed graph with doubled edges.
You want to list friends of Alice. Directed: query all friend(Alice, X). Undirected: query all friend(Alice, X) OR friend(X, Alice). Finding all friend(X, Alice) will be very slow without an index of all friend relationships with Alice in the second position. Storing this is harder than just storing friend(Alice, X) and friend(X, Alice).
More generally, in Facebook's social graph, connections can be of different types. For e.g., for a user, 'likes' can be one type of an edge and 'followers'(people he follows) is of a different type, 'followed by'(people who follow him) is yet another type, so on. Many of these types can only be uni directional(e.g., all the above three examples). So, the social graph is a directed graph. Of course, the well known 'friends' connection is a bi-directional type of edge.
I assumed, I think incorrectly with hindsight, that the 'friends' connection dwarfed all others and would be separately optimised. I think you're right though - 'likes' and 'friends' probably don't dwarf each other at a guess, and as Scaevolus pointed out it wouldn't be fundamentally different anyway.
The only efficient way to implement an undirected graph is as a directed graph with doubled edges.
You want to list friends of Alice. Directed: query all friend(Alice, X). Undirected: query all friend(Alice, X) OR friend(X, Alice). Finding all friend(X, Alice) will be very slow without an index of all friend relationships with Alice in the second position. Storing this is harder than just storing friend(Alice, X) and friend(X, Alice).