Scammers have found a simple way to fly under the radar during the early phases of their operation. The story they originally post to Facebook has nothing dangerous about it. It's just a regular story that anyone might post.
Only after the post gets a certain number of likes and shares does the scammer edit it and add something malicious. In fact, if you go back through your history of likes, you might find that some of them have changed to something you wouldn't have liked in a million years. So, what kinds of stories do scammers start with to trick people into liking and sharing?
One popular type of story is the emotional one. You've definitely seen posts showing you rescue animals and asking you to like if you think they're cute. Or maybe it was a medical story where you're asked to like that the person was cured, or to let them know they're still beautiful.
There are also the posts that ask you to like to show that you're against something the government is doing or disagree with something terrible happening in the world. Or maybe it's the "if I get X number of likes, then X will happen for me" or "I was challenged to get X number of likes," and so forth.
Basically, any post that asks you to like it for emotional reasons, unless you know the person, is quite probably a like-farm post. Of course, emotional posts aren't the only types of post you need to watch for.