Sorry, I'm bored so I'm hanging around here what does this mean?
I think it's a daily count but I'm not sure.
I asked Flyaway the same question a long time ago
Here is his answer
We have two "people online" counters - one for the forums which is availible on the forums index page at the bottom, and one for the whole of the Fly Away Simulation network portal (on the left of the screen on all pages). Both counters are very accurate - and represent users online over their session on the Fly Away site
It's the amount of users currently online browsing the Fly Away site.
Thanks. That satisfies my curiosity.