@brav I've always presumed it was the photos which have the highest number of views, favs, and comments in quick succession... (and this does seem to fit with the photos that make it there)! But I don't actually know for certain!
@brav I've seen the same photo hit the top twenty twice before too... and that time I did notice the no. of favs and comments had almost doubled... from the previous week. So I'm sure that's how it works!
I've been told by Ross that Top 20 is based purely on number of Favs for the set period of time for each week. it has nothing to do with pop page, views, etc.
@monkeykid - nothing to do with views, comments etc. those factors go into determining pop page. Instead Top 20 based purely on number of Favs for the time period of the week.
sometimes you can have enough favs to make the top 5 of the top 20 and get the favs within the timeframe in which the favs are measured and still not end up on the top 20. Its supposed to be purely based on favs, but I've seen enough pics posted in the right timeframe get enough favs to land in a good position in the top 20 but not actually make it from several users.