Ask HN: Does a sitle like vocabulary.com make money?

the site in question is : http://www.vocabulary.com/

It works well, gives you a daily quiz, and has achievements and graphs to keep you going.

I'm considering building something similar for my native language, when I started thinking about money.

I can't find ads, premium memberships. Yet I assume their user base is substantial. They also bought a great domain name. So do they have a business model? Or just a hobbyist on steroids?