How did these studies come up with such starkly different results? It all has to do with how you actually use Facebook. Are you actively liking, commenting and chatting with others on the platform? Or are you passively lurking?
Studies show active users get positive brain stimuli when engaging with the Facebook platform, while lurkers tend to feel increases in loneliness and dissatisfaction.
Image source and article: http://www.npr.org/blogs/alltechconsidered/2013/09/16/223052837/whether-facebook-makes-you-lonely-depends-on-how-you-use-it