Check the sources! Unfortunately, many people repost, retweet, and forward complete lies, mostly because either they WANT the message to be true, they are gullible, or in the case of a negative message, they want to feel like they are part of the solution by spreading the message, when in actuality they have been duped into being part of the growing problem of misinformation. The latter is an example of how people take advantage of the altruistic tendencies of others for their own personal gain or enjoyment.
A good site to check claims quickly is snopes.com. But even Snopes relies on other sources, so depending on the issue, you might want to check other sources as well.