9gag was immensely popular and profitable before joining YC. They joined YC for the experience.
They monetize the service by ad impressions. I can't think of any other ways they can more deeply engage with the audience while monetizing at the same time.
Previous class, 9Gag had been around for a while before they got into YC.
Most successful YC company if you ask me.
In what terms? If usefulness is taken into consideration I would rate Dropbox and Stripe considerably higher (and a few others).