For each friend in the app, you get a dedicated conversation and activity log page, where you can send instant text messages to one another, as well as see recent activity. It's here that you can respond to your friend with a snap by pressing the yellow button.
If your friend is viewing your shared conversation page at the same time you are, that yellow button turns blue. Tap and hold it to start a new video chat.
/ Photo: Screenshot by Sarah Mitroff/CNET