You couldn't have actually put a lot of money on it (unless you bet one of your friends, or in the unlikely case that you or your family member works for LinkedIn). The trick with IPOs is that even if you had the shares earlier, you couldn't sell them today because of the six month lockup period. The only way for you to get in would be right now at ~$80.
that is true if you want to directly purchase shares, but there were market makers and other outlets offering CFD's and derivatives at just above the list price.
judging by the volume (9M traded so far, which is more than what was listed) there seems to be a lot of that going on