Every estimate I've seen of the total number of bitcoin users is completely arbitrary. I think 625,000 is probably low.
I was curios too. If you have the patience to look all the way right and zoom in, you will see the source and date: 2/10/2014.
This stat is over half a year old.
And so much has happened in the last 6 months although I suppose it wouldn't change things too much.