Taken from minnow booster steem users tracker:
This of course does not mean a million people have joined.
(since many people have multiple accounts for various reasons)
It would be quite hard to determine when exactly million people have joined, even with the data steemit has from verification there are many people that join anonymously either for privacy or to bypass the free account waiting period.
That being said, I think anyone holding steem can agree this is a good sign. The growth of steem (or any system) is often a compounding percentage of increase over time, especially when the incoming traffic is from search engines and then creates more for the engine to index after joining.
This is the snowball effect of steem :)
So it will only become more popular in the years to come.
\ (^_^) /