Linguistic expressions in social media vary along many axes, including author style, the specific medium and its affordances, and others. In this paper, we argue that different registers must be distinguished within social media and that register should be included as an important factor independent of (social) medium in analyses of variable linguistic phenomena. We introduce a new German cross-media corpus, consisting of blog posts and tweets from the same 44 authors. We define the registers as ‘Informative’, ‘Narrative’, and ‘Persuasive’, based on situational characteristics of the texts. We then correlate the registers with two variable linguistic phenomena: German modal and intensifying particles. In each case, we document considerable inter- and intraindividual variation in the expressions used and their frequency across texts. The statistical analysis shows that the register grouping corresponds more closely to linguistic similarities between texts than the grouping by medium does.