Almost all the successful non-Google ad networks and exchanges are written in Erlang, Goldman Sachs HFT platform is written in Erlang, several massive scale game platform backends are written in Erlang, several international "national net" banking switches are written in Erlang, a huge chunk of the global land-line and mobile telco switching infrastructure is written in Erlang, Heroku's control plane is written in Erlang, GitHub's RPC backplane is written in Erlang, Amazon SimpleDB is written in Erlang, Bet365's high-scale betting platform is written in Erlang, Pinterest is migrating services to Elixir (rooted in Erlang).

There's a ton of stuff written in Erlang out there. A lot of really core infrastructure services that have to just run and run and run.

It's not particularly trendy, but it gets shit done, and it does so with a quiet dignity. :-)

RabbitMQ and CouchDB are written Erlang; Much of Ericsson's telephony infrastructure runs it; various companies in their industry like TMobile and Motorola use it.