Did you know that today is World Emoji Day?
Emojis have become an important part of modern writing systems, especially as used online. They may not (yet) be accepted as playing a role in a formal register of writing, but that does not mean that they are not valid written signs.
In fact, emojis are in some ways similar to ideograms, the term we use for individual written signs (e.g. in ancient writing systems such as Linear B) that refer to whole concepts. A smiley face conveys that you are happy or have been made to smile, but instead of writing this out in a sentence you can convey it with a single written sign.
The Linear B ideogram for a horse.