When something sounds to good to be true, it usually is!
I never trust anything unless it comes from the actual business’s facebook page. If you are not sure (and sometimes people do create FB pages that look legit), go to the businesses website and click on their FB logo to go to their FB page. I did this recently when someone posted a giveaway of Disney tickets. The FB page looked liked Disney (even down to profile picture and cover photo), but with only a few hundred likes and a “too good to be true” offer, it was suspicious — and, of course, it was NOT LEGIT!