My language of choice is Ruby, but I know because of twitter that Ruby can't handle a lot of requests. It is a good idea using it for socket development? or Should I use a functional language like erlang or haskell or scala like twitter developers did?
If ruby is your favorite language, yes it is a good idea. It is always better to use what you know and what you like
The key is to have a low level library in C/C++ that does the socket multiplexing for you. Socket multiplexing is what makes a TCP server process truly multi-user. such libraries in C (which is what you want) could be libevent/libev... and in c++ boost::asio. Python has twisted that does it behind the scenes.
If you get such a library and use it in ruby you should be able to implement most socket programs fairly well. This is especially true on UNIX oses which favour multi-process to multi-threading.
Whereas you may get better performance from a functional language such as Erlang the suitability of Ruby will really depend on what you are trying to achieve. For example how many requests are you going to be handling is probably the first question, if the performance benefits of using Erlang don't make much difference use something you are comfortable with, why learn a new language if you don't have to?
I'm not sure that any "lessons" that the Twitter team has learned about Ruby (more specifically, Rails) and scaling would apply to your project. They're looking at WAY more traffic than most people can reasonably expect to see.
As far as sockets and Ruby go, check out I like Unicorn because it's Unix. It's quite an interesting read about doing sockets in Ruby.
The company I work for uses Ruby for our web site. We have so far handled a little over 34,000,000,000 hits. We have no problem handling around 10,000,000 hits per day. Peak hits have exceeded 40,000,000 hits per day.
Scalability depends on a lot of factors. Our databases do a disproportionately high percentage of writes compared to reads, for example. While most websites do about 90% reads to 10% writes, we are closer to 50%-50%. My point is that scalability is affected by a lot of factors. If you are database-limited, as is often the case for web apps, it won't matter what language you use, you'll be waiting on your database.
There's a lot to think about if you are looking at handling large scales. Sharding databases, memcached, etc. etc. etc. etc. The language you use for your application is just one aspect, and often, though not always, a small aspect of scalability.
Ruby may be a good option for you, but there's a lot to like in other languages. Erlang tries hard to make it easier to recover from errors, for example.
You at least have the option of staying in your favorite high level language if you use a fast, concurrent language like Haskell, Erlang or Scala. With Ruby, performance bottlenecks will mean switching to compiled C (or Haskell, or ...) for speed anyway.
Ruby has the advantage of good frontend frameworks.
I'd like to provide a bit of context first. I'm pretty active with the Scala community, and I would choose Scala over Ruby for any project.
So, having said that, keep with Ruby unless you actually hit barrier. If Ruby is your language of choice, it might just be that you'll never be happy with the choices you mention, particularly the statically typed ones.
It might be good to learn a new language, to have something to fall back on if you need an alternative. In your case, I'd recommend Clojure or Erlang. Scala is a good statically typed, OO language with functional programming perks. It might be easier to learn than the others, but people who really like dynamic typing don't convert to static typing easy.
As for Haskell, it's one of the most awesome languages out there (and much more well support and popular than the equally awesome alternatives), and can open your mind like nothing else. It's also tough to master.
I have also used Ruby for many projects though I've recently moved to Scala and like it quite a bit. One thing that I've heard good things about (but never tried myself) for network stuff in Ruby is EventMachine. It uses the Reactor Pattern just like twisted and it seems quite solid.
Having recently written (actually still doing so now), a project using sockets with Ruby and Java I would say no. The ruby socket implementation is poorly documented unless you plan on writing a basic blocking chat server. I found writing in C or Java simpler, Ruby wraps up native sockets and your kinda left wondering how the hell to use it now. I have previously written plenty of socket code on windows, Linux and other platforms in C, with less stress.
My Ruby code now is very small and works well, getting to that point was a real pain.