I don't disagree with you, but to the parenthetical: I've unfollowed plenty of people. They're still in my friend list, but I never see their posts because they were too annoying. It's not strictly one-way like with Twitter, but it's close.
They're people who you want to be connected to for some reason (often a real-life connection) but you don't particularly enjoy their content. As opposed to Twitter, where you will follow people you don't know because they share content you enjoy.
The connection is often more formality than anything else (we game together, play soccer together, blood-relations, etc.). And they seem to like my posts, so they're following me, but I don't care for theirs and unfollow them.
There is also a non-negligible likelihood that those traversals go often through certain spam accounts that span strangers.
I don't see how. Even if you think the ties are quite weak, they undoubtably exist.
> In FB graph, I would think that vast number of relationships are not mutual, active or "leveragible".
Really? Unlike Twitter, I think Facebook actually emphasizes mutual friendships. (There's no one-way followers.)
I have around 1,500 friends on Facebook and that seems like a pretty high number. But I do know all of them, at least in passing, to the extent that I'd be willing to pass along a message for one of them.
Separation calculations aren't about extracting big favors, they're about finding minimal ties. One of the original experiments was literally just passing along a letter. [1]
[1] https://en.wikipedia.org/wiki/Small-world_experiment