Ask HN:Help for server side options. I know only .net

Hi all,<p>Myself and my friend have been discussing an idea for some time now. It involves location based social networking not very different from Foursquare. My friend is a mobile developer and I am a .Net developer at work. These are exciting times with Node.js out. I am also pretty proficient with all client side technologies like html5, css and javascript.<p>Though I only know asp.net as server side technology but we both can really take up any technology.<p>We'd like to get suggestions on what to use on server side - Php, .net, java\scala, ROR, node.<p>What do you think we should be using on server side? Please suggest. We'd like to start soon.<p>Thanking in anticipation.