It's been bothering me, was it only rich upper class men who would be the only ones literate enough to write infantile sentences like "We have wet the bed. I admit, we were wrong, my host. If you ask ‘why?’ There was no chamber pot.”, and was the intended recievers of this "information" upper class or learned men? I guess I'm also kind of asking how literate the population of roman cities like Herculaneum and Pompeji ind the first century a.d?
There was a related question a few days ago on which I wrote a bit about literacy in the Roman Empire which might interest you. In short, basic levels of literacy were probably far more widespread than one would assume, even though only 10-20% could be said to posess a level of literacy that we would deem 'literate' today (with a noticable gender inequality).
So there were many who would be able to read these graffiti (though impossible to say exactly how many), nobles as well as slaves, and not only the elite would have been able to produce these. So the potential audience is pretty large. I'm not an expert on graffiti, but these kinds of graffiti are still popular today, in bathrooms and toilets everywhere.The examples from Pompeii are numerous, and they display many topics which are still popular in graffiti (not the elaborate, artistic ones but the basic stuff that anyone can make). The name of a person, with who he or she was there, what they did, what they wanted to do, who they had sex with and how good it was, who was available for sex, what did they drink, which gladiators they supported, who they loved or hated, then there's poetry, sometimes original, sometimes quoted from famous authors. Often they seem not to have been adressed to anyone in particular, but simply a result of the desire to, literally, leave a mark by scratching your words into the wall where anyone can see them. People do the same today. Any social historians or anthropologists around who'd want to weigh in on that?