> Does this mean that a web server written in node is running single-threaded?

JavaScript is single threaded. Node.js is not.

Node executes the entire JavaScript code in a single thread. However, the I/O requests dispatched by the JavaScript code (file I/O, network I/O, db I/O) can be executed in separate threads.

That is why Node is efficient for applications with lot of I/O (typical web apps), and can still provide a default thread safety for global objects.

But I think, it will not work efficiently for number-crunching CPU intensive applications.