We are more than a million here in Bitcointalk.org and until now i am meeting people that has been involved in bitcoin but don’t even know this site so what i mean here?
Theres alot of bitcoin user outside bitcointalk so in totality there are many people using this outside our knowledge and by this Brian Armstrong is almost right now and what more after another 5 years(when people are more adopting computerized things than physical and this is the specialization of bitcoin
I agree with your opinion, one billion bitcoin users in 5 years. Most Bitcoin users are in developed countries, so users spread faster and multiply because the system that was born and developed in developed countries is very easy to adopt in developing countries.
Especially if the system is easy and generates benefits for users, it is certainly increasingly popular, so that 1 billion people use bitcoin is not impossible.