In-app advertisements have become a major revenue source for app developers in the mobile app ecosystem. Ad libraries play an integral part in this ecosystem as app developers integrate these libraries into their apps to display ads. In this paper, we study ad library integration practices by analyzing 35,459 updates of 1,837 top free-to-download apps of the Google Play Store. We observe that ad libraries (e.g., Google AdMob) are not always used for serving ads – 22.5% of the apps that integrate Google AdMob do not display ads. They instead depend on Google AdMob for analytical purposes. Among the apps that display ads, we observe that 57.9% of them integrate multiple ad libraries. We observe that such integration of multiple ad libraries occurs commonly in apps with a large number of downloads and ones in app categories with a high proportion of ad-displaying apps. We manually analyze a sample of apps and derive a set of rules to automatically identify four common strategies for integrating multiple ad libraries. Our analysis of the apps across the identified strategies shows that app developers prefer to manage their own integrations instead of using off-the-shelf features of ad libraries for integrating multiple ad libraries. Our findings are valuable for ad library developers who wish to learn first hand about the challenges of integrating ad libraries.