I think this is mostly a US thing. Why use yearly salary? You're not paid once a year, are you? Most likely once a month. Referencing monthly salary makes much more sense.
"I'm making 50k". Great, now I have to guess - dollars? Monthly? Yearly? If yearly then what's the monthly paycheck? Net? Gross?
People whose currency IS dealt in that scale.
For example, if you want to restrict it to English speakers, then anyone from Hong Kong would be flaunting a quite decent, but not millionaire, salary.