So I've been poking around Viral pages for the past day, here's my take.
1) Appears that only creations made with the meme generator get ranked on the Viral pages. I dont think creations made with the gif, pie chart or demotivational generators are in there.
2) Appears all memes including unsubmitted meme comments get ranked once a day, at midnight Pacific time. The Viral rankings don't change the way either Hot or Latest do.
3) Appears the ranking criteria for the Viral pages are number of views and how new the meme is.
4) When the daily rank is sorted, imgflip puts out 10,000 pages of viral memes.
imgflip.com/?sort=viral&page=10000
5) On high page numbers (like Page 9000) you can only use the back button, to go towards the front of the pile. On each high number page above Page 572, the "next" button at the bottom is grayed out.