Facebook had a relatively large number of private investors, enough to trigger additional restrictions regarding reporting and disclosure. Facebook hadn't intended to exceed that limit, but hit it anyway due in large part to new restrictions that prevented intermediaries such as brokers from letting many downstream customers count as 1, as well as people reselling their shares on secondary markets to extract value from them.
Those same private investors and brokers did quite a bit of trading when the IPO happened, much of it not to the benefit of Facebook.