Yeah, I didn't think they were literally using Roman numerals to describe their user base; I thought they were using the "10M == 10,000, 10MM == 10,000,000" notation that is (for some reason) popular with financial-type people[1].
But anyway, I was wrong; I poked around on the site and they indicate they really do have millions of users. So: wow, nice work!
I think andrewstuart was asking what qualified as a "user": how many were active, how much they engaged, etc.