Hey there! I was also on the waitlist for USC and just got the call this past Friday that I got accepted. I was offered either the OC campus or SD campus (first choice was UPC), but I quickly accepted the OC campus!
I had previously emailed the grad admissions office about their waitlist and they said they don't really have a numerical waitlist and they just plug in whoever fits the criteria of the person who withdrew.
Good luck with everything!!