High performance site
- Twitter uses Ruby-on-rails and Scala
- Facebook uses PHP
- StackOverflow uses asp.net mvc
As you can see, it doesn't really matter what you choose; all of these sites have lots of traffic, but are based on very different technologies.
What matters most in a social networking sites is the backend, since most of the bottleneck will be from there. You might want to consider No-SQL databases.
There are a few others like:
- Hypertable
- MongoDB, used by Sourceforge.
- CouchDB
As for the programming language, as others have said, it does not matter that much. But if you really can not decide, you might want to consider a non-blocking webserver like Tornado.
Doesn't matter what kind of scripting language you'll choose, as long as you'll heavily utilize memcached. Having the right caching hierarchy is a must.