Mark Granovetter is a Stanford sociologist, who wrote by some accounts, the most influential sociological article ever in "The Strength of Weak Ties". Keep in mind that the article came out in 1973- long before the Internet. The idea is simple and extremely powerful. Your close friends are those who know everything about each other and give you solace,comfort,support and joy in good times and bad times.The downside from a new opportunity point of view is that these friends know the same people as you do- so nothing new comes out during interactions with these "tried and trusted" friends. Sort of like the Jerry Seinfeld comment in mock despair : And these are my friends!
Granovetter found that unemployed folks looking for work during an economic downturn found job leads not from friends but "friends of friends." These second or third level connections had insights,contacts and leads that were very different from ones' own immediate circle.
Linkedin puts an entirely innovative Internet spin on Granovetter's original theory, making it hugely scalable by constantly telling you about people you "might know" in your second and third level connections. It is probably for this reason that recruiters find Linkedin useful and are able to reach more candidates when they advertise there. How exactly the individual job seeker gets leads from second order or third order connections is not clear,unless off course the individual keeps reaching out to connect with these individuals.
Either way the Linkedin IPO performance will offer great insight to all folks interested in the Internet and social media.