I think a lot of it is spiders
and general searches/hits from google however there are also thousands of active members that tend to live in blocks around the world. For example: I live in the UK so as I go to bed PF seems to liven up as everyone on the western side of the Atlantic starts coming home from work. It's also funny to note that there is a similar flurry from about midday when those same people check and post in the morning. I haven't noticed other flurries because I think (though I may be utterly wrong) that non-American membership seems more spread out across the globe.