Instagram could be considered a defensive acquisition. They should argue that the motive behind the acquisition was to expand their presence into mobile/imagery, which is different than defending social networking.
Whatsapp I have a tough time seeing as anti-competitive, since it's a messaging platform rather than a social network, and one amongst many.
The playbook is pretty clear for them: argue that they are in the digital advertising space, which is highly competitive, and argue that social networking itself is also competitive, with Snapchat, VSCO, Tinder and others as their competitors.