Distribution of clicks on Facebook posts

I’m probably missing something really obvious here, but I’ve just been looking at the clicks on bit.ly shortlinks embedded in posts on two Facebook Pages, Asda and TOMS. It seems that each can be plotted on really nice-looking curves:

asdaclicks

tomsclicks

Is that so obvious so as not to merit mentioning? Why should it be like this?

Please tell me what you think.