I think it is wrong to call bitcoin a "private" currency.
On the contrary, imo bitcoin is much more of a public currency that any national currency is.
It is not private because anyone can use and can have it, bitcoin is a public currency because there is a lot of people who can earn and invest in bitcoin because in bitcoin you can earn even you do not have regular job because here is different even you do not use it in a month you can still have it and because are promoting as a pay system. (I guess)