Yeah... pretty much what Devy said. Also, Bonfire is a MUCH smaller area. So in truth, Flourite has more people, just more spread out.
I mean, think of it like this:
Take 10 marbles. Put them in a paper cup. The cup looks ridiculously full, right?
Put 15 marbles in a bathtub. It looks like NOTHING, correct?
Smaller areas have larger population densities, if that makes any sense.
Either way, no matter WHERE you go... you can always find people to chat with. Even Last Cave has a good group of floofs there every once and a while. ;)
Fluorite Plains is generally the more populated map. However, the difference between the Plains and Bonfire is the size of the map. Bonfire is pretty small compared to the Plains which is rather big, leading to the Plains being more sparsely populated with people spread out more, and Bonfire being more densely populated with a lot of people huddled together in one place.
Of course the time of day and year is a thing to consider to, like Sura said. More people are likely to be online during the summer holidays and half terms when less people have school rather than the autumn and spring terms when people have started the new academic year and prepare for exams. Weekends are a little busier as well, for obvious reasons; people don't tend to go to work or school on these days and have a bit of time to chill and relax.