Ever since the American documentary Catfish stirred up controversy in 2010, "catfishing" has become a widely used verb for those using social media in a deceptive way. Usually this involves a "sock puppet account", where someone takes another person's photos and takes on an invented internet persona.
This is often used for romantic purposes, a scam to trick unsuspecting users into engaging in flirtations and conversation online. The places that these people get their bogus profile photos from tend to be from Facebook itself, where you can download photos that people have put up publicly.
While you can make your profile private, that doesn't stop those who are allowed to view it, such as friends, mutual friends and family, from taking the photo themselves. It's unlikely you'll ever come across their fake account, so many think it's a safe way to live another life on the web.
Up until this point, there didn't seem like there was much we could do about it, but thankfully, Facebook have come up with a new solution that may just solve the issue entirely.