In this study, we examined how users strategically leveraged the two affordances, content persistence and ephemerality, for self-presentation and relationship maintenance on Instagram. In contrast to other social media platforms that usually support either persistent (e.g., Twitter) or transient content sharing (e.g., Snapchat), Instagram affords both functions through regular posts and short-lived Story. We conducted a seven-day diary study on Instagram users (n=13) and used the data as probes for follow-up in-depth one-on-one interview. Our findings show that people deliberately curated persistent posts to present their ideal images. In contrast, ephemeral Story shows more authentic and casual sides of self. With the interpersonal and ephemeral touch, Story creates more private and directed engagement among users and their audiences than the interactions derived from posts. Story also helps users develop deeper relationships and extended interaction with others without experiencing "context-collapse."