What do i build my alpha platform on, PHP, Ruby, Python, .NET, etc?
It doesn't matter.
StackOverflow was written in ASP .NET MVC and it's awesome.
Twitter was written in Rails and it's super popular.
Facebook was written in PHP and half a billion people use it.  
It's not the technology, it's the community.  That's the hard part.
Just pick one and go.  Your best bet might actually be to find the technology that the most smart people are using while still working for the least amount of money.
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.