londrum - 8:15 pm on Jun 14, 2011 (gmt 0)
What this means is that the people considered active (no statement as to how many that is or how they are measured) are really active and visit the site every 2 days on average.
that might not be as good as it sounds. what about all those bazillions of 'like' buttons that are all over the web -- in iframes? if a user visits a site with one of those on, then presumably that would count as a visit to facebook too, even though they'd been nowhere near the place.
and the same thing with all those comment boxes that people paste all over the place