A social network is fundamentally a community of people.
On the Internet, this community can contain people of various attitudes and behaviors; behaviors often more uninhibited than in the physical world.
In this way, structure applies more to the constraints of the community instead of the design of the website. Structure comes in the form of rules and enforcement.
Which comes first? The site or the rules? Or are they considered in tandem?
How are the rules communicated to the community? Are the rules over-restrictive for the purposes of the many personalities involved?
Do the rules address real issues, or just a collection of “over-the-counter” problems that may not pertain to the goals of the social network in question?
Who is assigned to interpret and enforce the rules? Is it one person? Or is there a collective of differing mindsets adding their own personal criteria to address rule compliance?