Is there a specific term for sentences written with symbols or emoji only instead of words? An example would be a string of emoji on a facebook comment of a post.
A pictorial symbol for a word or phrase. Pictographs were used as the earliest known form of writing, examples having been discovered in Egypt and Mesopotamia from before 3000 BC.
Note: The answer given by @vickyace is still technically correct as pictograms, or pictographs, are a form of ideographs/ideograms.