Stylistic variation in online social media writing is well attested: for example, geographical analysis of the social media service Twitter has replicated isoglosses for many known lexical variables from speech, while simultaneously revealing a wealth of new geographical lexical variables, including emoticons, phonetic spellings, and phrasal abbreviations. However, less is known about the social role of variation in online writing. This article examines online writing variation in the context of audience design, focusing on affordances offered by Twitter that allow users to modulate a message's intended audience. We find that the frequency of nonstandard lexical variables is inversely related to the size of the intended audience: as writers target smaller audiences, the frequency of lexical variables increases. In addition, these variables are more often used in messages that are addressed to individuals who are known to be geographically local. This phenomenon holds not only for geographically differentiated lexical variables, but also for nonstandard variables that are widely used throughout the United States. These findings suggest that users of social media are attuned to both the nature of their audience and the social meaning of lexical variation and that they customize their self-presentation accordingly.