How old are your managers? judging from your brief conversation, they seem rather immature, perhaps they're in their late twenties. You work in retail, your job is to sell the clothes they offer to customers, so you're going to wear clothes that are targetted towards teenagers and young adults, such as casual styles; Jeans, tank tops, graphic tees, skirts, denim jackets etc. Even though they're basic, they're comfortable and they're on trend. Plus everyone behaves and dresses differently outside of their job, when you're not working, you can be completely yourself and you can relax, but when you're at work, you're focused on making sales, and meeting certain goals and criteria it's a completely different atmosphere. It was uncalled for-for them to imply that you weren't looking decent, they must pride themselves a lot on their appearance if they're going to bring someone else down like that and act snobby, so ignore those remarks from them, just humour them, nod to what they say so that it looks like your agreeing with them when you're not.
To be honest, even though they are your employers, what exactly were they trying to look for? Where are they trying to dig up anything that could put your job in danger? were they hoping to find some info that they could expose you with? Because it seems to me like they fell flat, and they could only find that you're a normal girl, using facebook like everyone else. They had no business to stalk you like that and invade your online privacy if you haven't given them any reason to mistrust you or be suspicious of you.