A large set of mobile applications (apps) heavily rely on services accessible through the Web via REST APIs. The mobile apps market is therefore becoming a key market for service providers. However, the way in which mobile apps use services in practice has never been studied. In this paper, we perform an empirical study in the Android ecosystem in which we analyze 500 popular applications and 15 popular services. We also conducted an online survey to identify which are the best practices for Android developers. Our results show that Android developers generally favor invoking services by using an official service library instead of directly invoking services with a generic HTTP client.Our study shows that Android developers prefer to use a dedicated library developed by the service provider if it exists. The main advantages of such a library are that it simplifies the calls of the REST services but also provides added-value such as Android Views for example.