So far at Plurk we have struggled to overcome scalability issues - - and none of them are due to Python, even thought we use Python a lot.
Most of the problems we have seen have been related to the database. It was and still is the biggest bottleneck. And all the people I know that drive big sites will confirm this.
This all said, one should evaluate problems and not blindly use Python for everything. Python is a great language, but some other language is a better choice for some problems. And here Java and specially Java's NIO library is a much better choice for doing a server that should handle tons of open connections.
Most of the problems we have seen have been related to the database. It was and still is the biggest bottleneck. And all the people I know that drive big sites will confirm this.
This all said, one should evaluate problems and not blindly use Python for everything. Python is a great language, but some other language is a better choice for some problems. And here Java and specially Java's NIO library is a much better choice for doing a server that should handle tons of open connections.