Facebook’s flagship messenger has ridden out the storms of protest that followed Apple’s exposure of its alarming data collection and forced change of terms. But WhatsApp has gone further, warning tens of millions of angry users that they risk their private messages being read. Signal surged after WhatsApp’s backlash, fueled by media headlines, but that rush of installs has faded. WhatsApp has been single-minded in its defense since January, focusing on security credentials on which it built its brand. Facebook CEO Mark Zuckerberg said back in January that iMessage is a key linchpin of their ecosystem.