Its the age-old chicken or the egg question - what came first? And in this case, the debate is regarding whether or not Facebook creates jealous people, or attracts already jealous people to their folds. Case in point: a recently released Canadian study found that the more time users spent perusing Facebook, the more jealous they were.
Now, the sample size was small (just over 300 people), and used a young age range (17-24). But the 'Facebook jealousy scale' that they created seemed very, very real, dishing out questions - such as how much of the green eyed monster a person felt when their significant other befriended someone of the opposite sex that they didn't know. However no conclusions were drawn regarding whether or not Facebook created jealous people or not, saying the options were "inexorably intertwined". I'd be interested in seeing a study showing how jealous you are before using Facebook, and then measuring jealousy levels again after signing up.
Source: Time Magazine, Could Facebook Be Sowing Seeds Of Jealousy?