You know very well there's no way to meaningfully quantify desktop numbers so I'm not quite sure why you're asking for them.
Since you seem to want me to be pedantic, this is what I meant by "shockingly popular": I thought MATE would stall out after a year or two but I'm shocked that its releases are still coming steadily and rapidly, meaningful commits are landing, the forums are lively, and they've made some real architectural fixes." Overall, qutie impressive. Does that make sense?
So, allow me to ask again... What more will it take for you to notice that MATE is "becoming more popular over time"?
I guess we must agree to disagree on whether Gnome 3 was a pretty experimental release. Was there much user testing that I'm not aware of? If so, I'd love to see the reports.