Where does classmates fit into the picture? I don't think too people actively use them, but some people do. I'm actually unsure how Classmates actually works

Where does classmates fit into the picture? I don't think too people actively use them, but some people do. I'm actually unsure how Classmates actually works --is it a subscription model, what's it's business model and could it end up morphing into the TheFB of old but subscription based? Maybe the draw of advertising money is too strong to resist.