EDIT: Because of changes in iOS 4.3.5, you can now install the Emoji keyboard without installing a Japanese keyboard. For the new directions, please go here:
http://markystar.wordpress.com/2011/08/30/using-japanese-emoji-on-facebook-update/
People keep asking me how to do this, so I figured I should just make a blog post and make the details available to everyone. After all, it takes seconds to set up and will make your Facebook posts standout all the more.
Because Japanese uses Kanji, an ideographic writing system, it’s only logical that including pictures or emoticons along with text would be a natural thing to do. All Japanese cellphones come loaded with tons of emoji (絵文字 “pictographs/emoticons”). There are sets specific to au, Softbank, and DoCoMo (the major cellphone carriers in Japan). But there is also a universal set that is cross platform (and even displays on computers).
When the iPhone was released in Japan, immediately people started complaining about how boring it was because it didn’t have emoji. After a few months, Apple released an update to the iPhone OS that among other things, brought universal emoji functionality to the Japanese input systems. People in other countries don’t know much about this feature (and other awesome iOS features) because they are peculiar to the Japanese input keyboards.
I’m not sure if there’s a way to do this via the computer (but seems like there must be since the emoji are displayable on computers). If there is, I don’t know how. Also, the other drawback is, you’ll have a Japanese keyboard pop up as one of your options when you go to do text. You can arrange the order in which keyboards pop up when you have multiple keyboards installed, so putting the Japanese at the very end is a good way to keep it out of sight if you don’t need it.
STEP ONE
Go to Settings

go to the settings icon
STEP TWO
Go to Keyboards.
Turn on Japanese.
don’t worry, it will keep your previous language. you can install as many as you like, i think.

add the Japanese 10 key input
STEP THREE
Enable Emoji.

it used to be disabled by default, but i'm not sure about recent versions of iOS. might be pre-enabled these days. anyways, you're a big boy. you can figure it out.
Congratulations! Now you can use emoji just about anywhere… including SMS, e-mail and Facebook.
Here’s a screenshot from the Facebook app for iPhone and I’m updating my status with text and emoji. To toggle between keyboards, just tap the International button in the bottom lefthand corner (it looks like a globe). Use this toggle button to get back to your regular language input after you’re finished using emoji.

shock and impress your friends with your emoji arsenal
You can use the emoji in any app that allows for them. All the native iOS apps, including Messages (SMS), e-mail, etc. allow emoji use. Here’s an example of emoji used in the Messages app.

your mom
Here’s a list of all the characters that you now have at your disposal. This is how they display in iOS. So your iPad and iPhone will look like this. On Facebook, they will display differently. I don’t have a list of those. Sorry.

this image borrowed from inner.geek.nz
If you want more technical background emoji, then please visit this website:
http://inner.geek.nz/archives/2009/02/06/the-truth-about-iphone-emoji/
He’s got the skinny on the inner workings, which I don’t.
Have fun with your emoji!
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