Google Trends is a great free tool that allows you to analyse search trends over time. It can help you spot seasonal patterns (e.g. 'Christmas'), but also compare results.
Here is a comparison for Facebook vs. YouTube:
You might want to get students to hypothesise what a graph might look like then test it out. It allows you to see searches from specific countries which might help get a grasp of world news or current events. For example, the trend of searches for 'ISIS' shows the speed in which the organisation has grasped media and public attention:
Even if you don't find any direct pedagogical applications for your subject, it's still pretty interesting!
Click here to go to Google Trends.