Limits to Predicting Online Speech Using Large Language Models

We study the predictability of online speech on social media, and whether predictability improves with information outside a user's own posts. Recent theoretical results suggest that posts from a user's social circle are as predictive of the user's future posts as that of the user's past posts. Motivated by the success of large language models, we empirically test this hypothesis. We define predictability as a measure of the model's uncertainty, i.e., its negative log-likelihood on future tokens given context. As the basis of our study, we collect 10M tweets for ``tweet-tuning'' base models and a further 6.25M posts from more than five thousand X (previously Twitter) users and their peers. Across four large language models ranging in size from 1.5 billion to 70 billion parameters, we find that predicting a user's posts from their peers' posts performs poorly. Moreover, the value of the user's own posts for prediction is consistently higher than that of their peers'. We extend our investigation with a detailed analysis on what's learned in-context and the robustness of our findings. From context, base models learn to correctly predict @-mentions and hashtags. Moreover, our results replicate if instead of prompting the model with additional context, we finetune on it. Across the board, we find that predicting the posts of individual users remains hard.