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Whether you think of it as a currency or not does not mean they are taking the money out of the US economy. They are using it it to buy things here in the US, it is still very much a part of the US economy.
Investing is putting wealth into things that you think will grow in value and produce new and more wealth for you.. So yes putting money into bitcoin is investing.
People are buying bitcoin in part because they expect the value to rise. That is investment, full stop.
The value is rising because of its functionality, and once that functionality becomes more common place then it will cease being an investment for most and instead act more as a currency with a stable price and new functionality current fiat does not have. But we arent there yet.
Some people are buying it because they dont trust the US economy I'm sure.
