What do i build my alpha platform on, PHP, Ruby, Python, .NET, etc?
Write in assembler if you're comfortable with that.  :)
Some questions you should ask yourself:

Are there hosting restrictions? No point in coding ASP.net when you have a PHP-only host/server.
Are there technical restrictions? E.g. if you want to use SQL Server as a back-end, going with ASP.net may make your life easier.
What other requirements do you have? Does it have to run on the JVM? Do you want to compile stuff all the time or do you want an interpreted language? etc. etc.
What experience do you have? If you're already familiar with Python, why switch to Ruby?

My best tip is: use what's best for the job at hand according to the above questions. For me, I'd use Ruby on Rails for the project you described. Rails offers all the tools I need for a large project like that. 
Please let us know when and what you've decided :)
I am developing a social networking website in the facebook/foursquare-ish space. I have gotten such varied feedback on what platform I should develop in. Of course it will be heavily influenced by who I hire, but i was hoping for a little additional feedback from the larger community. Thanks.