This paper presents the findings of our study into the performance of two popular instant messaging applications: WhatsApp and Skype. We have evaluated the performance of WhatsApp and Skype in terms of their data consumption/usage when sending text messages and making VoIP calls, as well as the quality of the VoIP calls made with these applications. Our results show that WhatsApp uses less data compared to Skype when sending text messages and making VoIP calls. We also find that both applications have similar VoIP call quality (mean opinion scores) when network conditions are good. However under poor network conditions, WhatsApp VoIP call quality is better than that of Skype. Our results will be useful to smart phone users who have mobile plans with limited data allowance, and provides guidance to them in terms of their selection of which instant messaging app is more suitable for them.