Could Telegram, Whatsapp or FB run using Elixir?

Well considering WhatsApp is written in erlang it would certainly be doable in elixir as well, elixir and Erlang have nearly identical characteristics being based on the same VM.

Elixir is amazing but Telegram has like 30 million people in it now and since Elixir isn’t exactly performance-oriented I am not sure it can handle the load without spinning a lot more servers which might not be economically viable.

At this scale you definitely should reach to something like Kafka + Rust.

Are you sure Elixir can perform equally to Erlang in those use cases? I am tempted to go with Erlang to be safe, but wanted to get a more definitive answer before deciding not to use Elixir.

Throw enough hardware at it, and sure, no problem. You might need to pull some fancy tricks to be able to take advantage of that hardware efficiently, but that sort of thing is actually one of the things that Elixir is good at, leveraging Erlang’s distributed processing, supervision trees for reliability, and so on. It might need more hardware than something programmed in raw Assembly, but probably quite a lot less than something stood up in, say, Rails.

Is Elixir or Erlang a better choice to run in this use case and why??

It is possible to write bad elixir, just as it is possible to write bad Erlang. But all the tools that Erlang gives you are also available in elixir, so you can definitely use elixir to write anything you could write in erlang.

Is there ANYTHING that would prevent Elixir from performing (this use case) as well as Erlang?

They both compile to BEAM bytecode, and Elixir essentially evaluates to the same Erlang code, so no.

You mention Telegram, Whatsapp or FB as example of scale. Discord is using Elixir and it’s massive.

No, not really. They compile to a very similar (or exactly the same) internal bytecode. Erlang IMO has no innate advantage here. That’s the whole point of the BEAM bytecode if your compiler is good (and Elixir’s is) then you can have an identically performing code no matter the source language.

I might even be so bold to say that if you’re building a commercial product on the BEAM you should probably pick elixir by default. Before any erlangers get mad at me let me defend myself. The reason is that it’s easy to use Erlang (or any other BEAM language) library from Elixir, but it’s harder or impossible to use a lot of very good libraries like Phoenix and ecto from Erlang due to elixir’s macro system. If you’re building something open source it might make sense to pick Erlang by default to attract the widest audience (see telemetry as an example). This is the elixir forum so I am of course biased, take this with a grain of salt.