We <3 Lisp! A couple of our engineers have past experience working with Common Lisp, Scheme and Elisp, and since we are also fans of functional programming, we have been really excited about Clojure.

We currently have two different web services running in production on Clojure 1.2.1, using Ring, Jetty, Moustache and Yammer's excellent Metrics library (see https://github.com/codahale/metrics for details).

Learning how to deploy apps on the JVM was the only real hurdle, since we don't have any Java experts on our team, but after some initial hurdles it was all sunshine and rainbows. Having a real concurrency model, an amazing JIT (Hotspot is pretty incredible), and a generational garbage collector means that we can write much less code compared to Python, but with amazing performance.

Of course, you could get some of those same benefits by using Jython or IronPython, but Clojure's immutable data structures, STM, Protocols, Multimethods, Macros, and concurrency primitives like Actors and Agents provide for an extremely compelling programming model.

http://clojure.org/features

In the end, we were able to deploy new features with significantly less code that required less hardware to run at scale, and got more comprehensive monitoring via JMX for free.

Watch our Github account! There's going to be some Clojure stuff there soon.

https://github.com/formspring

We use mercurial for almost everything, although we have a couple of internal projects that are in git repositories.

We currently have 12 engineers, but we need *way* more. Send us your resume! http://about.formspring.me/jobs

Yes we do! We are going to be releasing a ton of code soon.

https://github.com/formspring