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Well those two words are different to each other.
Mass adoption is when it is almost normalized to be used/utilized as an alternative or primary entity.
Mainstream adoption would be when you see/hear it almost in everyday discussion or you could see it in every media channels. It could be from your television, radio, social media platforms, etc.
One example I could think of for mainstream adoption but not mass adoption would be when you can almost see BTC price equivalent to USD in your local news along with every currency.
Mass adoption is obvious.
