I was also wondering this, and I have seen the per-server page, but it's tricky to compare.
What i want to know is:
What is the typical number of level 80 players online at 20:00pm
this is because I am thinking of transfering, and want to find raids at the time I play. On my current server, it seems like everyone has 8 alts at level 80, so the overall population is not representative. I want to know actual numbers online during the time I play. (Ideally also filter by level, for example for vanilla fans who would want to know which servers have a larger number of 60s...)
Ideally, I want a filtered overview, eg:
players level [80] online at [20:00]
server ------------alliance--------- horde---------
EU-Aszune--------1946-------------1017----------
EU-Moonglade----1342-------------859-----------
EU-Mug'thol-------839--------------768-----------
EU-Nagrand-------1913------------1072----------
obviously, i do realise that there will be quite a few servers with no-one supplying data....
[edit: whoa ghost posting, sorry]