50 million active users is a beautiful dream but impossible to achieve in a short term. I believe that with hard work it could be done in 5-10 years.

Currently there is 1 538 987 registered accounts (source) Based on data which I could find, I believe we can easily assume that only 10% of that accounts is active, which gives us a bit over 150 000 users.

To have 50 million active users, we'd have to invite half a billion people here. It's 12,5% of all people around the world who has the access to the internet.

I believe airdrops is a great idea though. Maybe part of the money from mining directly could be dedicated for that.