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.

The base tech of a "friend discovery network" isn't "hard" in the grand scheme of things. But getting those who don't put much thought into their tech to care enough to move out takes a gargantuan effort. Musk had to go full nazi to start seeing the bluesky adoption, and it still isn't the level of catastropic effect you'd think would happen if you heard about this 20 years prior.