I had a ticket open about this a while back and here are the answers I got:
Only looks at like and is calculated over all time (not by month).
This is content from the last month or less if there is enough content to fill the list.
Popularity is determined by calculation of activity with each type of activity having an assigned number of points (point assignment is unknown). Content becomes popular when you rate or like it. Popularity affects whether you see that content in the most popular content widgets found in Your View (your personalized home page). The more people who give a 3- to 5-star rating, like, or even view a piece of content, the more popular it becomes. Popularity can help place focus on content that is exceptionally useful. These explain what is used for determining popular content:
- bookmarks < included
- shares < Not included
- follows < Not included
- document ratings < included
- views < included
- likes < included