I use it in telecom (VoIP). At peak, we were doing 1 billion calls a day. All routing decisions, F#. All call records, F# (on Mono).
I've also used it in network indexing, processing 5TB of SIP signalling a day on a single server, then indexing and storing each packet for later searching. This is now used in the voice arm of a large public telco to provide troubleshooting.
F# is a joy to work with and every time I use C# it's annoying.
No, honestly, the biggest issue was just remembering to install the F# libraries.
Actually, the biggest issue is mono-service. Since we run cross platform, we used the Windows service API instead of writing a normal daemon. This has been a source of "random" issues with getting it to start. But that's just our fault for not changing 10 lines of code and just running it like every other Linux daemon. Similarly, some code depended on file locking (because FreeSWITCH's CDR writers don't properly write to files) which doesn't work on any popular Linux file systems. There are probably some other little issues where we made a dumb Windows only assumption.
All in all, it's been smooth sailing. Stuff just works. We develop and test exclusively on Windows, then deploy the binaries to Linux. I can't say enough good things about F# and about Mono.
Also, I don't use it on Linux, but F# Interactive would only work on some installs. A problem with readline iirc. I'd guess that developing on Linux will be more rocky than using Visual Studio.
I've also used it in network indexing, processing 5TB of SIP signalling a day on a single server, then indexing and storing each packet for later searching. This is now used in the voice arm of a large public telco to provide troubleshooting.
F# is a joy to work with and every time I use C# it's annoying.