The same thing happened to me a few years back. I got a product that came with an invitation to a secret Facebook group, with promises of further discounts. I joined the group, and it turned out to be a system where they would send you Amazon gift cards to buy specific products in exchange for five-star reviews. Apparently a large number of ordinary customers were involved, given the size of the group in question (all people using what appeared to be their real Facebook accounts).

I reported it to Amazon customer service, and they gave me a $5 account credit for making the report. Not sure if any action was taken against the seller.