> PHP sites such as Wikipedia probably need more servers (since every request is a separate process).
Citation needed. But every request isn't a separate PHP process; PHP is linked into Apache. Apache starts up a pool of processes and reuses them for requests. It's all highly efficient.
> By the way, Facebook uses Erlang for its performance-critical parts (such as chat).
Actually, it's not so much the performance critical parts but the parts where PHP's share-nothing architecture isn't appropriate. I doubt they're using a front-end web server for chat either.
> You could still use an agent-like messaging system for shared-nothing multithreaded/multiprocessed environment.
If you had the need, I suppose you could. Although I'm not sure what point you're trying to make here.
> When using cURL you must poll the handler until the request is complete. Obviously you'd need multithreading to achieve a better control flow in such a scenario.
You don't really need multithreading. JavaScript has callbacks for everything, for example, but isn't multithreaded. So this isn't so much a problem with PHP as it is a lack of design of cURL. But then cURL is a C library.
> I've written and debugged some wordpress plugins, and I can't say I came to the same conclusion as you...
Wordpress is old and not well designed. (Much like PHP itself)