I think you write it like a compare and contrast kinda thing.
So first do a short introduction.
Then write how you would talk in a FB Chat conversation and what kind of expressions you would use and how you would convey these expressions.
Then write hhow you would talk in a spoken conversation, so facial expressions, posture that kind of thing and also write about the way you speak to your specific audience. Like if you were talking to children you would speak slow and happily but if you were in a serious interview you would have a mature serious face on.
Lastly write a short conclusion about the ways you speak in both situations.