The IPO wasn't the first chance to cash out -- Facebook was being heavily traded in secondary markets before the IPO (and the lockup preceding the IPO).
The price in the secondary markets seems relevant here. Facebook's valuation was probably highest at the time of the IPO, and also the secondary markets are going to have downward price pressure because it's much harder to trade shares.
The IPO was the big chance for people with equity in Facebook to finally cash out and actually make some money off of the company.