In North America and Europe, realm population information is extremely difficult to come by.
You pretty much have to rely on WarcraftRealm's CensusPlus UI Mod (Link). Right now, for example it's shows a total of 5.5M characters (however, these are characters from Level 10-80 and each account can have a maximum of 50 players across all realms.)
So, you can understand that the numbers aren't that accurate.
Blizzard also doesn't share Realm Population information because of obvious reasons (those details can only hurt them, not help them.) Even the definition of Low, Medium, and High Population realms can change - there's no fixed number as I have stated previously.
Blizzard has even confirmed this to be true. The populations are relative, meaning that they can shift back and forth at any time, depending on the population of other realms and their relation to the servers (Source). Population caps can even be increased/decreased depending on the number of servers being utilized or out of commission.
That troll from Blizzard even had something to say on this specific matter (before I attached Blizzard's confirmation, of course): Is it made up like most of the other "facts" on this site? (Blizzard just confirmed it to be a fact Mr. Blizzard Troll. I guess that's further confirmation that he's just a support agent, and doesn't actually know anything about Blizzard's internal operations.)
China Realm Populations
So to summarize, in NA/EU it's almost impossible to determine census information because Blizzard will not share the information.
But in China, it's operated by NetEase so it's a totally different story! They're completely open to sharing player information on each realm. =]
Census information is pulled straight from the WoW servers by Beijing Network Technology Ltd. (there's no Census UIMod needed on each user's PC), and is publicly available here. Below are the numbers during peak gaming hours on WoW. In this case, there were approx. 790,000 users logged in concurrently.
Alliance: 338300 - Horde: 451655
So basically, a 3:4 ratio which is not bad.
The number of realms in China has also been growing steadily (many new realms launched this year and last), and some Chinese players have noted that many people are now returning in CWOW in anticipation of WOTLK's imminent release.
However, the first thing you'll notice is the incredible imbalances on the realms. I've attached screenshots below.
Some servers have 1200 horde and only 40 Alliance on the server. That's a 1:60 ratio! These drastic differences have also been confirmed by CWOW players.. they'll have thousands of players on one faction, but less than 50 on the other. It's not uncommon at all.
There also doesn't appear to be a lot of realm balancing activities. As you can see, realm A:H ratios are all over the place and most realms operate with one side significantly bigger than another. Because of this, it turns most realms into a PVE (everyone working together on one side) type realm.
To confirm my assumption, I checked with several CWOW players. They said that PVP is not very common in China, and it's mostly PVE. A more recent comment even stated, "even on PVP servers, PVE is much more popular."
As opposed to NA/EU Realms, the CWOW gaming culture seems to favor "Playing Together" rather than "Playing Against Each Other" (on average). There are many realms with a 1:0.01 ratio for example. And that's very accurate information (as hard as it is to believe for US players) and can be confirmed by any Chinese players playing on those realms. Server populations have actually improved quite a bit bit over the past few weeks, which is a good sign for WOTLK. =]
It's all pretty neat actually, and also explains why Guilds and gold PUGs are so huge and why gold farming is more popular and strategies used are almost identical. (For example, rare gold farming strategies that are applicable to NA/EU might not work in China or players probably aren't even aware of them.)
Based on those figures, I have to wonder if the same is true for NA/EU realms? When talking to US players, they often claim that their servers have 1:2, 1:4, or even 1:10 ratios (either they out-number the opposite faction, or vice versa.) They never had any evidence to back up their claims though, since realm population is not shared by Blizzard.
