i would say holiday 2015.
PS3 and xbox 360 population will devistate the PS4/xbox1 population like its a fucking joke dude. it will take YEARS before the PS4 population gets close to the PS3 population. xbox1 probably will never match PS3 until PS3 dies off, same with wiiU but for different reasons. PS4 will be lucky to have half of PC's numbers online.
id expect ruffly 10-30k online on next gen consuls at any given time once it levels out. about 300-400k on current, 400-500k after level out if the game actualy ends up good. about 20-50k on PC. There probly be 50k points on PS4 simply because there isnt very much to play on the system till the library is built up some, which will take time.
PS4 and Xbox One numbers won't be real big until 2014-2015. Around this time, Developers will stop supporting PS3 and 360 and gamers will have to choose to either keep playing their old games on their old system or to move to the next system. This was the same with the current gen. It took a year or two for them to become the prominent system. The PS4 however will have better numbers then the PC community. The pre-order numbers on PS4 have surpassed the numbers you assume PC will max out at. Also, parents will likely be breaking down and getting the new systems and games for their children, which will boost the PS4 and Xbox One numbers.
Still, numbers and what not aside, numbers won't be as important as they were in the past. Dedicated Servers will help make sure that we can have good, stable connections. So, if you're worried that to play you will have to be playing with people from across the world, don't worry about that. My clan and friends for the most part are moving to PS4 and so am I, and we know that we won't have to worry about getting horrible connections.
It's VERY easy to say that the Xbox 360/PS3 will have more than the next gen consoles will. Not everyone has the money to throw out of the window.
I would assume the PS3/360 would have a higher amount of people.