There's no such thing as a free lunch.

So, when you see an ad on Facebook saying that you can get a free Big mac, you just might believe it - or want to.

Here's the catch: you get this supposed freebie just by sharing and commenting.

The post claims every person who shares and comments in the next 24 hours will get three large big mac meals for free.

But the fact-checking site Hoax Slayer says what it's actually doing is spreading links to fake McDonalds pages. If you're sharing the posts, you're opening up your friends and family to scams.

MORE: Scam info from Hoax Slayer

The bottom line: if you see a post for a coupon you hope is real, go to the company's page first.

If the company is running a special, it’ll likely be posted on their page.