We see them regularly; lawyers who get you out of parking tickets, fad diets, horror stories from cosmetics usage but a lot don't hold up under scrutiny. Best example I have is a post that turned up on a group page showing a terrible looking infection claimed to be caused by Dove beauty bars. A few minutes researching however realized that the grotesque looking image was actually a seed pod that had been recoloured in photoshop. Parody sites can be just as bad. You often see posts from The Onion that people have assumed to be real from the Facebook link because they haven't bothered to click through and check it out.
While in the Dove example, there was obvious malicious intent involved, that's not how it ended up on my news feed. It ended up there from someone who genuinely believed that it was real and that it was something that people should be alerted to. These get perpetuated quickly because it's an easy way for people to feel like they're helping to improve society (similar to Slacktivism).
What can we do about it? Not all that much unfortunately. You can improve your news feed by letting friends know when they've shared something that's false or misleading, in private messaging preferably. But the most effective way to counter them is to not engage with them. It seems like you're helping by pointing out a falsehood in posts, but this act will cause it to turn up on the news feeds of friends and if they aren't as careful as you then you've just made it worse.
Wikipedia is better and less dangerous than Facebook. Even if the article can't be trusted, it usually points you in the direction of literature that can be.