In the case of true close friends, you know about all the crap that is going on in their lives. From deep interaction, you know the specific pains and doubt that lies behind the smiling profile picture.
No, the life-comparison danger comes from the weak ties; from those people you met at a conference, or the friends from High School that you haven’t interacted with since they friended you last year. From these people you get a constant stream of life, edited to show the good parts.
Since TV was invented, critics have pointed out the dangers of watching the perfect people who seem to inhabit the screen. They are almost universally beautiful, live in interesting places, do intereseting work (if they work at all), are unfailingly witty, and never have to do any cleaning. They never even need to use the toilet. It cannot be pschologically healthy to compare yourself to these phantasms.