The reality is that buying on Facebook at prices much lower than the official ones is basically a red flag. It's not that they're all fake, but the odds go up quite a bit. What most people don't mention is that besides checking with the official platform, you can also ask the seller to send you clear photos of the barcode and the ticket details, and then you can try to validate them yourself on Ticketmaster's website or wherever. If he says he can't or makes weird excuses, there's your answer.
Get in touch with the concert organizer or the legitimate ticket platform and explain the situation, giving them the ticket code if you have it digitized. They'll tell you in seconds if it's registered in the system or if it's a clone. Some let you do this even before the event, and if it turns out it's fake, at least you have proof of when you found out in case you need to file a complaint or report the seller later.
Anyway, even though it sucks, think about whether that money you paid is something you can afford to lose without it ruining you. Sometimes it's worth just buying a new legitimate ticket from the official platform, even if it costs more, than showing up on concert day with a ticket that doesn't work. And if the verification goes well, then at least you can relax.