Why do people let all their guards down when going onto Facebook?

For the umpteenth time, I've seen yet another "Like and share this image for a chance to win this item" post.

Usually from a big name retailer - Argos, Currys, Best Buy.

First of all - if it's too good to be true, it probably is - and that is definitely the case when clicking 'like' can win you a TV.

Secondly - two seconds of looking around tells you it's a scam straight away!

The one I've seen today is from "Argos." offering a 50" TV if you like and share an image.

If it wasn't obvious enough that Argos doesn't end with a ".", joined Facebook on the 4th October, and only has one post, I don't know how it becomes more obvious!

Why do people become so gullible when going on Facebook?