Platform as a Service (PaaS) is one of cloud service models associated with effective system operation in the presence of a rich set of platform services. Due to the complexity of the PaaS service model, software architects need to face various challenges when designing cloud applications that have to meet certain quality guarantees. In this paper we report on our experience with the design and implementation of highly scalable PaaS cloud applications, which resulted into deep understanding of the weaknesses and benefits of this service model, and helped us to identify and evaluate a number of architectural tactics for PaaS cloud applications. Besides the identification and evaluation of the tactics, this paper studies the mutual relationships and the impact of the tactics on various aspects of application's quality. The findings are demonstrated on a real-world case study of a complex private social network system, based on the Microsoft Azure cloud.