I see this all the time in my work place. I'm an average weight person, I see the skinny people mocking the fat people and the fat people mocking the skinny people, it is really some sort of cruel joke. Both spectrum's offer different health complications, whether you want to believe that or not.
I fully agree with you about the teen girls complaining about their weight. At their age they should be eating a lot of food to allow their bodies to grow and mature.
For an adult it is a completely different story. You can still be a high body fat % if you're 5'3 and 115 pounds. They may look skinny and healthy but a lot of women that weight aren't really "healthy" the healthy ones are usually around 125 lbs and have adequate muscle mass + lift weights. Yes that 10 pounds makes a HUGE difference.
There is no way to turn fat into muscle, and "toning" is a myth. You tone up by reducing body fat.
Many people have eating disorders, self-esteem issues and various under-lying causes that make them feel the way they do about their weight, regardless if you perceive them healthy or not. I'm just going to stick with my first statement and say that you're just extremely bitter.
I'm just going to chop this down to; if you don't think the a person needs support, don't support them. It's as simple as not looking at their post if you don't agree with it. Focus and love yourself, don't hate on other peoples life-styles. The other option is to re-direct them to healthier options - which I prefer to do.