The page accessed from the "X users online" link (http://projecteuler.net/index.php?section=activity) is pretty basic.
Might it be better to see some stats about the online users? I'm thinking of the same kind of thing you see in the 'Scores' pages, perhaps as a subsection much like the 'country' ones.
As well as just satisfying curiosity about what kind of users are more often online etc it's also far easier to scan down a list for a specific username than to trawl through the current setup.
Just a thought,
gyppo