Many of the stragglers were too young to learn to read at anything more than a very basic level. But for those that are capable of communicating in writing, and those that can find rare goods like paper and writing utensils, they often amuse themselves by keeping journals or reading the scraps they can find. If anything, it gives them a chance to find temporary escape from their troubles.
While most stragglers do not bother to communicate with letters, some trade written thoughts as keepsakes. Shadowy Hegemon figure may give orders in the form of a letter, and fortress survivors frequently use it as a means of communication.