I think it's just counting the players who have logged in from the website using the Play Game feature.
This is easy to test simply by logging in/out and refreshing the frontpage. When you log in, the counter will increase by 1, when you log out, the counter will decrease by 1. The more times you repeat it, the more sure you can be that the counter wasn't affected by random user logging in/out.
I tried to log in & out 10 times and refreshing the frontpage, and based on it the thing I said makes sense.