This issue is caused by Facebook’s in-app browser, which may not be storing the cookies needed to keep you logged in.
1. Log into your Facebook account.
2. Tap the Menu icon in the top-right. (It looks like three horizontal lines.)
3. Scroll down and tap “Settings & Privacy.”
4. Tap Settings.
5. Scroll down and tap “Media and Contacts.” It’s at the very bottom of the list.
6. Toggle the “Links open externally” setting to On. (It will turn blue.)
That’s all there is to it. From now on the Facebook app will load all links to other sites in your device’s default web browser instead of in the slimmed-down in-app browser. Unfortunately, this doesn’t work in iOS, only on Android devices.