With a normal web presence, it looks the same no matter how many people are visiting it.

With Facebook and other social idioms, it will be clear from the first pageview if noone showed up or if everyone left. The page/group's point in the hype cycle and demographics are obvious too.

Users can identify a failed launch or a ghost town at first glance, intuit that there's nothing there, and click away -- all without looking at your content.