Snopes.com, officially the Urban Legends Reference Pages, is a website covering urban legends, Internet rumors, e-mail forwards, and other stories of unknown or questionable origin. It is a well-known resource for validating and debunking such stories in American popular culture, receiving 300,000 visits a day.
Snopes should be your first port of call whenever you read a Facebook alarm about something or if you think something is a hoax. Check it before sharing any posts that sound a bit dubious. It also makes for very interesting reading.