What's the Population of Panem?

This question is closely related to the geography of Panem, but unfortunately, we don't get as many clues to base speculation on, especially if you define canon narrowly. We know from the first chapter of The Hunger Games that there are about eight thousand people in District Twelve. Katniss tells us that the Capitol is a city, and speaks about Eleven being a large district, speaking in front of crowds, etc., but doesn't provide any other numbers. In chapter 15 of Mockingjay, Beetee says he's been 'playing with numbers' and believes death toll of the rebellion may put Panem in danger of killing themselves off. While that's not as helpful as an actual number, I think it implies that the total population of Panem is near to a threshold of a human version of minimum viable population, or at least, a minimum viable population for maintaining Panem's current level of technology (since there's a significant difference in the number of humans required to maintain a basic, subsistence farming-based society and what we think of as a 'modern' society).

( click for speculation )