There are 2 sections to show who is online, the one that is normally open shows who has been online in the last 60 minutes, well apart from the invisible ones.
The other one you need to click on that double arrow that is pointing downwards on the right hand side of the picture, this opens up who has been online in the last 24 hours, if you hover your mouse over the user names it tells you the time they were last online.
Both the above will have the time spans made shorter in the future, ie the 60 minute one will most likely be brought down to 30 minutes and the 24 hours one down to 12, but thats for the future.