“Memory Trace Upload was successful” message randomly appeared on iPhone
Apple and iOS do not have that as a standard dialog or feature - Apple crash reports and memory traces are logged and controlled much differently than how the Facebook App is designed. 


https://support.apple.com/en-us/HT202100
https://developer.apple.com/videos/play/wwdc2018/414/
https://developer.apple.com/library/archive/technotes/tn2151/_index.html


Apart from the link above on whether you have Apple share analytics you report back to the developer, you can inspect your iOS device in settings to see what the Facebook app is logging to Apple and possibly through them to Facebook when you opt in to Apple’s analytics sharing process. 


https://support.omnigroup.com/ios-crash-log/


Open Settings.app.
Tap Privacy, then scroll down and tap Analytics.
Tap Analytics Data.
Locate the log for the app that crashed. The filename will use this format: &lt;AppName>_&lt;DateTime>_&lt;DeviceName>
Tap the crash log to open it.



Assuming you have discovered this message is in-app diagnostics and not iOS / Apple diagnostics and opting out from Apple collection, uninstall the Facebook app as a quick fix to remove the memory upload message that it is sharing with users when it it letting you know a trace has left your device. 

You would of course need to use Safari to access your feed until the app developer either explains to you why they are letting you know a trace is uploaded or opt out of analytic / memory trace collection within the Facebook app.