Roleplay means exactly what you think it means. It's a roleplay and in this case with kpop idols (usually). You act out as a persona/person and you are that person. Like in a play, there are different characters that need to be played, say you get a role, you take on that character.
After that, it basically becomes Improvisation. You respond to what the other people say. For example, I'm Taemin and I see Key. We know that Taemin an Key know each other because they are in the same group So I would be like "Hey Key how are you?" Then the person playing Key would respond how he/she chooses. And I would play off of what he said. So you just make up scenarios with other people using your chosen idol.
As for the facebook account, you don't always have to, but a lot of them are done through fb, so thus if you wanted to join in, you'd have to have a fb. Some are twitter based and idk where others are lol
As for always needing members, its because people join and don't interact much and they need more to keep it alive or its new, or they are just always welcoming more, you can never have too many in a roleplay lol
As for it being interactive, is that you are actually the person being the member, you decide what you what your character to do, someone else is not dictating you. You have (mostly) freewill to do as you please with your character.
does that make sense?