I understand that one as a reference to facebook being a social network, rather than a reference to its actual management structure.
That makes sense.
They used to have a similar picture on their front page, IIRC.
It could represent a bottom's up engineering culture, but the graphic would be missing junior engineers and leadership which would add more tree like shapes into it.
Flat management structure with small autonomous collaborating teams.
Facebook's homepage used to have an illustration of people connected across the world that looked like that.