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.
