These things always have snowball effects. As it increases, it'll increase quicker.
MattyD, one thing i'd suggest for this case study, is instead of seeing just "how fast a silly viral page grows", you could analyse the effects of viral advertisement, showing how fast the growth accelerates. Take regular notes of how many members the page or comments has at timed intervals. You can then see how something viral may start slow, but grows at an ridiculously increasing amount!
You'll find once it hits the million mark (if it does), odds are it will increase faster than before it was at that milestone.
These things give perfect data for visual elements, line graphs or whatever to show the video's growth in popularity (I'm assuming this is for a school/college/university project?)