I've never signed up.
The whole thing seems really naff especially the way it encourages users to cherry pick happy perfect moments from their life to post.
The mental health effects of social media are a whole other question which is just as troubling in my opinion. Pretty sure studies have shown a timeline correlation of the rise of social media alongside the rise in mental health issues especially in the young. http://www.center4research.org/social-media-affects-mental-health/
Constant comparison with others "best self" is a recipe for increased unhappiness.