Social Media: Avoiding scams and hoaxes

We've all seen the posts on our favourite social networks - endorsed by friends and relations that tell us about a new startling discovery/scary news item/unbelievable photo/medical marvel...

Such things used to be (and to some extent still are) the domain of chain emails, foolishly forwarded and rather irritating at best.

But these days we are getting these posts shoved in our face by those people around us who 'like' or comment on such things.

So what are these things, what is wrong with them, and how can we spot/avoid them?

Here goes...