Published Information on Effectiveness of Duolingo

I have been looking for information on Duolingo:

1) http://techcrunch.com/2014/02/18/duolingo-raises-20m-series-c-round-led-by-kleiner-perkins-wants-to-dominate-online-language-learning/

This article states that Duolingo has 25 million registered users and 12.5 million active users. Apple chose Duolingo as the 2014 iPhone app of the year. 

2) http://static.duolingo.com/s3/DuolingoReport_Final.pdf

This research paper is about an 8-week study carried out on learners of Spanish in the USA, learning Spanish on Duolingo. They found that the motivating factor behind studying had an effect on how well students studying, with those reporting that they were learning Spanish for travel doing the best. Other factors such as age or gender had no effect on the rate of language acquisition by means of Duolingo. They found that Duolingo was effective, especially for beginners, and that 29-46 hours of Duolingo was equivalent to the first semester of Spanish on a US university course.

3) http://www.quora.com/How-effective-is-Duolingo-in-learning-a-language

This is a forum, started by the founder of Duolingo, Luis Von Ahn, with many personal testimonials from language learners who have used it. I would like to pick out the following quotes:

"It is gamified in a way that I have found very efficient and addictive. You get "sublevels" (lessons), "worlds" (units), trophies (when you finish a unit)." 

"It is auditory - unlike a book, the iPad speaks to you. You speak back to it. You record your voice and check your pronunciation. You do it again." 

"Programs like Duolingo are a game changer - we are only seeing the beginning of what kind of impact these can have on language instruction and almost any other topic." 

"I tried Duolingo to learn Spanish and german, and I can say it worked fine. From a knowledge zero I could read easily El Pais (one of the main spanish newspapers) in 6 months for instance."

4) http://www.forbes.com/sites/jordanshapiro/2015/01/08/duolingo-for-schools-may-change-the-edtech-market/3/

This is an article in Forbes Online about the Teacher's Dashboard feature dated 08 Jan 2015. It talks about the fact that Duolingo now has 60 million users, and compares it to Facebook in the early day: millions of users but as yet no way to make money. They mention how detailed Duolingo is in analysing data, and how well students are learning, even timing how long it takes them to answer each question. I would like to share the following quote:

Imagine what could happen when you put the power of that kind of data in the hands of a classroom teacher. “The goal is to provide a personalized learning experience that gives each student and instructor immediate feedback in the classroom,” says Luis von Ahn. “This can free up teachers’ time to concentrate on difficult concepts, answer questions, and assist students falling behind.” Now imagine what happens to education when you make that kind of technology available to teachers for free.