I fell victim to this scam in Oct 2019. I switched 2 lines - each one was eligible for $200 rebate (total $400). I was told by the sales rep to expect the cards in about 2 months. When my cards did not arrive, I called customer service - they informed me that I had to enter my information in some website within 30 days. Initially, I was told it was too late to do anything.
After multiple calls and many hours, Verizon customer service credited $100 per line (total $200) to my Verizon account. The Customer Service Rep said this was the best she could do. I find this hard to believe and I am still $200 short.
I like your suggestion of posting about this scam on social media. I think contacting the Better Business Bureau is also a way to go.
Note - one of my calls was transferred to the 3rd party vendor that issues the gift card. I don't understand why I was transferred to them - they could do nothing without a reference number from Verizon. I found it interesting that the person I spoke with was upset about the huge number of calls from people with this exact same problem.