I really like how you can see how many people are learning each language now. Scroll to the bottom at look at "Incubation Phase 3".
Yup! Spanish is the language with the biggest number of learners. It currently stands at 2.4 million, followed by Spanish users learning English (2.3 million).
English is actually the language with the most number of learners. You gotta add up all of the totals.:)