Writing on social media often departs from prescriptive norms through the use of non-standard words, spellings and punctuation. Amongst these traits is the repetition of letters (e.g. <ouiiiii> for oui ‘yes’). In this study, we draw upon a corpus of over 65 million tweets from three dialects of French (Laurentian, Metropolitan and Midi) to test phonological motivations for the choice of repeated letter in a word with repetition. Using mixed-effects multinomial regression, we compare dialectal differences in whether repetition targets final consonants (silent or pronounced), word-final orthographic <e> corresponding to phonological schwa, and prosodically accented penults. We demonstrate that repetition covertly signals phonological properties. We conclude that prosody mediates morphological and phonological effects and that grapheme-to-phoneme correspondences vary between regions, thereby producing phonological patterns that writers likely did not intend to convey at the time of writing. We also propose that orthographic repetition on Twitter has two prosodic sources: the default pitch accent in French (shifted or not) and focus.