Well for one, most of the girls that you see on Facebook that are half naked, in lingerie, or in small panties are from Facebook pages that you liked or someone on your friendslist has liked a picture from that page so it pops up in your newsfeed. Most of them do not even know that their pictures of them are out or leaked to the public. So basically it is not them that are posting it up. People think it's really the girls themselves that are actually posting their own picture and so the guys literally try and talk to them through the comments, they're so dumb seriously.
A lot of pages save/download pictures that are already leaked out on the web (forums, google images, and so forth) and post them up without the women's permission. For example, these two people were in a relationship together and they were sending pictures to each other (sexy or naughty ones) and once the relationship ended they posted up their ex pictures all over the web. Yeah, that's how sh*t gets all started. Some of them could be models, some of them actually want their picture taken like that so they can show it to the world (they find excitement in that, being seen by random people from all over the world), some of them just wants to be sluts and hoes (it's true), and others.
Well the girls that you do know who post pictures like that they're either 1) insecure about their body so they show more skin to get more likes and compliments so they can feel that everybody loves them 2) proud of their body so they show it off. Their body, something they can actually control. It makes them feel wanted and desired even if it's not the way they wanted to be desired. It draws the wrong kind of attention but some of them don't care, as long as they feel desired is all that matters to them. They want attention, they seek attention, they crave and yearn for it, attention is what drives them.