Completely unscientific but as a rule of thumb there are typically 10% of my Facebook friends online at any given time. Except for early in the morning, anecdotally this stacks up with what others have said as well.

I am online for most of my waking hours though not necessarily always on Facebook. Assuming I am on 16 hours a day, which is a bit of a stretch but not wildly off the mark this suggests that my friends are typically on Facebook for around 1 1/2 hours a day.

I’m not always active when I am online, of course.