If you like to "Like" stuff on Facebook....you need to be careful! A Swiss man was just fined in a multi-person libel case (which includes the original posters of the content) for just liking a post. He didn't add or post anything else but just hit the "Like" button on several articles on a website.
Similarly to the people who were sued for libel for retweeting a false news story a few years ago.
Though those in the US have not had anything like this happen, don't rule it out. With the plethora of "Fake News" being put out on social media - it is bound to happen. Especially since it has happened elsewhere in the world.
The crux of the case is when you Like or Retweet, you are "publishing" something to people who may not have seen it before - duh!
In both cases the platforms themselves are not libel for anything their users post though both have had issues with "colorful" topics removed from the platforms.
The future legal ramifications could be interesting. So be careful out there in what you post, like, tweet, or retweet - it could come back to bite you later.