It's not unusual for deadlines to be changed. It looks like the FTC filed a motion for an extension a week before the initial deadline (6/23) and Facebook didn't oppose.[1] (Typically opposing counsel will discuss and agree before these motions are even filed so it's not surprising that Facebook did nothing in response to the motion.) The linked press release makes it sound like the Commissioners voted on whether or not to re-file the complaint so I'm guessing 30 days wasn't enough time for the FTC go through whatever that governance process entails.

[1]https://www.docketalarm.com/cases/District_Of_Columbia_Distr...