> This number is pretty deceiving.
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