When you send messages on your iPhone, Android phone, or via Facebook Messenger, these messages are saved in the respective databases of the corporations who make that software. As mentioned in the “Crossing Borders” post, Apple, Google, and Facebook regularly comply with government requests to hand over user data. When you are sending sensitive data, i.e. any and all data that you do not want to be public or used against you by the government, stop using these apps and tools to communicate. (We could argue that this encompasses all your digital communications.)
Instead, use an encrypted texting app. We recommend Signal. It’s free, relatively easy to setup and use, and highly endorsed by many prominent online security experts and activists. You can send texts, group texts, photos, video, and make voice calls to anyone with a phone number.