Thanks for stopping by the Apple Support Communities. If I understand your post, you've noticed that while looking at pictures on Facebook, your iPhone X display seems to flicker on the left side. I am happy to help!
If the issue is specific to Facebook, and does not happen in other instances, try the steps outlined here: If an app unexpectedly quits, stops responding, or won’t open - Apple Support. Even though the title of the article doesn't match your issue, the steps inside are still quite helpful if an app seems to be misbehaving.
Should you find this flickering to occur in other instances outside of Facebook--such as in other apps, on the Home screen, or in Settings, try reinstalling the iOS software. You can do that following the "Put your device in recovery mode and set it up again" section of If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support. Like the previous article, the title of this one may not match your issue, but the steps are still relevant.
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Screen flickers
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