Apple introduced a feature in late 2013 that allows users to download the last compatible version of an app if they have previously "purchased" it on their Apple ID.
For those with a jailbroken device, you can manually install .ipa files found in web archives like Archive.org . facebook messenger ipa for ios 4.2.1 download
Because the native app often fails to connect to modern servers, the most reliable way to access Facebook Messenger on iOS 4.2.1 is through the . Navigate to: m.facebook.com Apple introduced a feature in late 2013 that