Messenger does 3 things right:

1. Being able to discover people by name / surname, no phone number necessary. This is the most important privacy feature people care about, it's ironic that Meta had it from the get-go, while other platforms have barely caught up.

2. Used to have frictionless message sync, including in the event of a catastrophic loss of all devices, which put it far ahead of most apps (sadly nerfed by E2E).

3. A much better group implementation than Whatsapp / iMessage (no need to maintain a contacts list, no need to share phone numbers with everybody, you know who everybody is by name and surname). This is perfect for semi-professional groups where people are acquainted but not close with each other, especially when some members hold positions of power and don't wish to receive calls from irate people). Parents / teachers or blue-collar coworkers are perfect examples.

It's sad that all these apps are converging on the same set of features and mis-features, with nobody (except Telegram) really exploring the tradeoff space any more.