Every estimate I've seen of the total number of bitcoin users is completely arbitrary. I think 625,000 is probably low.
Yes, there are between 1-2 million unique coinbase wallets and 99% of these are only US clients. Extrapolating the different exchanges there are at least 4-5 million bitcoin users right now.
The data wasn't even accurate 6 months ago either as coinbase passed 1 million back in feb and most bitcoin user live outside of the US. This would indicate that conservatively 3 million was the approximation back in Feb.