I misspoke, mine actually came out at 2:30 PST like shockadeli (I was just estimating before I hadn't actually checked). So it seems like they sent them all out at the same time, but I could be wrong. Hopefully you get one if there's more coming!
They asked us to RSVP by December 26 so that they could know the number of interviewees and make sure not to invite too many. Considering they haven't rejected you and their website says you will receive an invite by end of December, I'd assume they will send out more interview invitations if not everyone who got one chooses to accept it.