The Facebook app only supports one account that is logged in at a time – but what if you need one for the job and one for your personal life? This usually means logging out and logging in every time you need to switch accounts, but your OnePlus device offers a much better solution.
The main reason you can only have one active Facebook account logged in per session probably has to be from a security and privacy standpoint. They do not want to risk having a handful of accounts running on the same active session as they can be easily accessed at once.
As I said, to get started running two separate Facebook accounts on your OnePlus phone, go to your system settings and go to the "Tools" section. Find the "Parallel apps" section and dive into these settings to see more information about the feature.
You get a list of apps currently installed that are supported by the feature, which will probably be filled with social media apps. . Find the "Facebook" option and switch to On mode to create a "Parallel" version of the app. This means it clones the most important Facebook app to create a separate instance of it, essentially giving you two different apps. Cool!
When you switch to your app drawer, you should notice two versions of Facebook app at this point. The new "Parallel" version is highlighted with the orange parallel app icon at the bottom right. Open the new version of the app, and you can now sign in with another account so you can use two Facebook accounts at the same time.
If you ever wanted to delete the parallel version of the Facebook app, go back to the settings page "Parallel apps" and turn the switch to the off position. You will receive a message letting you know that all data will be deleted when you disable the parallel version of the Facebook app. Confirm your selection by pressing the "OK" button and it will be removed from your system immediately.