The succses of the "social graph"

A friend of mine works for Groupon, a website that offers deep discounts on services and products, but the discount will only take effect if enough people purchase the deal. (Sarah blogged about this site earlier.) While I found out about the website from my friend, it seems to have been spreading, particularly via Facebook. I recently took advantage of a Groupon for dance classes, only to find that two other friends (with whom I had not shared the link, not knowing if they would be interested) had purchased the Groupon as well.

Did these friends find out about the Group on because I (or another dance class-loving friend) had posted it on Facebook, or did they learn about it through some other non-social media channel? I am sure that the Groupon website has something working in the background to determine where their successful click-throughs come from, but would it go so far as to show that people clicked off of my specific post?

I am concerned with this question, not only because of the privacy implications, but also because it indicates an overlapping of the "social graph" and the "real-world graph" that I was not initially buying into. I was convinced that the "social graph" of social media sites was so thin and overreaching that it could not possibly meaningfully influence others' purchasing decisions like "real-world graph" friendships could. I thought - and, to a certain extent, still do - that marketers' move to rely on the "herd mentality" was naive: if two people are not really anything more than one-time acquaintances in real life, why would their "friendship" on a social media site prove to be any more influential in making purchasing decisions? Then I realized that one of my friends who bought the Groupon is really more of a casual acquaintance; maybe the trick of the "social graph" is not the extent or depth of the interpersonal relationships but rather just the additional brand or product exposure.