Purpose: Libraries worldwide, including Hong Kong, increasingly use social media tools to introduce and promote their services and resources to users. Instagram, in particular, is used to target younger users. This study investigates the effectiveness of Instagram in promoting library services and university students' perceptions of the value of Instagram as a learning support tool. Design/methodology/approach: A major academic library in Hong Kong was chosen for this study. Library users' habits, perceptions, preferences, and views on Instagram's effectiveness as a learning support tool were compared in two age groups. The data were collected using a survey based on the 5E Instructional Model. Findings: Despite the significantly higher frequency of Instagram use by younger students, the results showed that Instagram was probably an ineffective promotion platform for either age group because of low user engagement, relatively neutral perception of Instagram as a learning support tool, and notably low user acceptance of Instagram as a promotional tool. Originality/value: Studies of student perspectives on various social media tools have increased; however, few have explored the use of Instagram, especially in Hong Kong or Asia. This study provides researchers and librarians with practical insights into current Instagram users' engagement, perceptions, and preferences and their view of its effectiveness as a learning support tool. The study also provides suggestions for improving the current situation. Copyright © 2023 Emerald Publishing Limited.