  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.
