For anyone looking for it, this is on the Account panel under the drop-down which shows your name and language pair in the top right of the screen. (In Web, obviously.)
This is about as minor a bug as it’s possible to have, but this section is all about bugs so here goes.
When you turn your visibility OFF by UNchecking the box, the confirmation dialog warns that it will make your stats “visible”… which is of course the opposite of what happens:
Conversely if you check the box to make yourself visible again, you are warned that it will REMOVE you from the rankings (or, as it is spelt in the dialog, “ranings” (sic)), which is again the opposite of what actually happens:
I’m sure that most users who have found the dialog understand how things hang together well enough to recognise the dialogs as an error, but they may be confusing for complete newbies.