So, yes, we may soon see some sort of a Facebook coin, but it might be just a glorified voucher.

I doubt it. If Facebook was going to award loyalty points of some sort to its users, it would have done so well before Blockchain. Adding a crypto-aspect to an already useless feature won't make it more appealing to anybody. Facebook is either going to be offering a coin in a new and unique way that we won't be able to predict, or they'll be operating in a space that won't be directly customer facing.