Who are they, besides Joe? Can you at least provide their names?
I'd really like to take a look at their credentials because beyond WhatsApp and an obscure Ericsson router from 20 years ago, there isn't much data to go by.
And there is plenty of evidence to show that C, C++, Java and more recently Go have some pretty strong success stories to justify their claim to scalability.
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http://erlang.org/faq/introduction.html
Amazon SimpleDB Delicious From: http://stackoverflow.com/questions/1636455/where-is-erlang-u...
Also many of the companies using Riak (written in Erlang) also leverage erlang. Some pretty big names use it.
http://basho.com/about/customers/
I would say that Erlang has more than proved itself.
One aspect of it is that it is used more of an infrastructure language so not many people see it as glamorous as rushing out to the Go community to brag about their new backend. Its a lot of mature companies that have been around a while and don't really need to show off their infrastructure.
Also Elixir is gaining momentum (but still a blip) as a very viable language built on erlang.
Use what you want and what works.
Ah yes, the "dark matter" argument. It's out there but nobody can see it. You'll just have to trust me.
As for listing companies that use Erlang, plenty of companies use PHP and COBOL too. This doesn't say anything about whether these languages are the most adequate for the task and whether these projects wouldn't be in a better shape if written in a different language.
One of them is here on HN: https://news.ycombinator.com/user?id=rvirding
But, "practical" doesn't show up on everybody's radar much these days with so much effort focused around fads-of-the-half-decade.
Erlang is made by professionals with decades of experience who grow the system towards being the most stable, scalable, safe, and as understandable as possible. People use Erlang, solve problems, stay quiet, then make money using it.
On the other hand, the thought leader these days seems to be Go (even though it's made by out of touch weirdos (review: http://www.evanmiller.org/four-days-of-go.html)) and has a creepy cult/fanatical following more suited to tribal sportsball teams than what we would expect from professional technical work needing to be stable over the long term.