A long time ago, the Apple ID was like any other username. But nowadays Apple requires all Apple IDs to be verifiable email addresses. Since everyone's Apple ID is an email address, many of these have been found in data breaches and leaked online. While your actual iCloud or iTunes accounts may not be compromised, the email address you use for your Apple ID could have leaked along with billions of other account records.
iPhone, iPad and iPod touch owners have been warned about a FaceTime bug that was recently discovered, which could affect privacy. In a nutshell, FaceTime callers can hear audio through your iOS device's microphone while it's ringing. The would-be eavesdropper has to follow a few steps to unlock the bug, however anyone receiving an incoming FaceTime call could be at risk, before the call is answered.
If you use Instagram you probably share your Stories with family and friends, and probably don't want them shared with people you don't know. Your friends and family, however, can share your stories with others as direct messages. If you have your account set to private it doesn't matter because only your followers will be able to view it, even if it is shared as a direct message.
Siri enjoys delivering answers to iPhone owners whether their devices are locked or unlocked. Sometimes the call "Hey Siri" is even answered no matter who is asking. Pressing the Side button or Home button can also invoke Siri from the Lock Screen. For those who consider this a privacy or security risk, Siri can be completely disabled when the iPhone is locked.
Facebook, by default, is set up to send you notifications via SMS for just about everything that can happen - comments on posts, friend requests, birthdays, friends' updates and more. The notifications seem to be random as well, sometimes there are none for a while, then all of a sudden a flood. Depending on how active you are on Facebook, these text notifications can quickly become a nuisance. If you suddenly find yourself being bombarded with text notifications from FB, you can turn them off completely or better yet, pare them down to only what's important to you.