[phmqk76]

Google Plus had 100 million active users. Wouldn't have called that mainstream either. Apple's iMessage has 1 billion MAU - that's the kind of scale that Apple can achieve, instead of being a niche novelty for most people, or as others have brought up, the go-to tool to write essays for English 207.

[doktrin]

So in your view 100 million MAU is not mainstream while 1 billion is. Where is your cutoff? Absent some criteria you’re just arbitrarily defining goalposts.