users online

can i see somewhere a statistic about number of users online(by days or hours)?

In game, no you can't see.

in the game,obvious,no,but is there any other site which keeps this statistic?

Only per month you can see, when you try to search for application on facebook, you will see (8m players active montly) or something like that.

There is no website who count how many players log in daily.