Reciprocal relations between role players in social networking

What I've been thinking much about lately is how 'reciprocity' (you give something you gain something or, conversely, you get something you gotta give something) works in the online networking world. Facebook in particular.
Who all contribute to Facebook? What do they get in return?
Who gains from the service? What do they give in exchange?

So, looking at the various role-players involved.

• There are the founders and ongoing creators of the site. Mark Zuckerberg and those who work with him.
• Then there's 'civil society' - the users. All ~150mil of them - can imagine there are quite a range of motivations and uses there.
• And business - advertisers, partners (notably Microsoft), those investing in the service for future profit motives.
• And when we have civil society and business, the natural flow of thought leads to the state sector. One can pretty much guess what government agencies could gain from a massive and growing database of identity information willingly submitted by users. What are they prepared to (/ do they) give in exchange?

I'll be exploring these ideas further in months ahead.