Google Produktmanager interview questions

Was introduced to a few positions through a friend that works at Google. Was contacted by HR to schedule a phone interview. Interviewer called a few minutes late and we started the interview right away. He was a PM from China, working at Google for 4 years already. He asked me what Google three google products do i like. When i said one of them was gmail, he asked me to say what i like about it. Then, when i said the free aspect and the large storage amount, he asked me how can i estimate the amount of gmail users in the world. Further questions were on which features would i add to gmail.