Uh oh. Can I add my opinion?
As an American, I say "wanna" most of the time. As someone who makes her living writing, I would never use "wanna" in a business communication. I use it in chat and I use it in casual e-mails to my friends.
It seems to me that people come to ESL forums to learn the type of English they need for examinations or work. Therefore, it seems to me that when writing,efforts should be made to comply with the rules of written English, unless we're specifically discussing pronunciation.
So I support trying to keep writing like "thanx 2 u for ur help" and "if u cud help me, i'd be happy" -- no matter how friendly the tone -- out of posts that talk about grammar. (And support talking about how we often leave letters out in actual pronunciation.)
Just my two cents.