Multiple threads can definately listen to single socket. All threads can be blocked on "select" or "poll" system-call. Now depending on number of CPU your that many number of threads will come out of select/poll system call then if its TCP you mandatory call "accept" system-call. Here only 1 thread will succeed and remaining threads will fails which again go back to select/poll system-call. Now if instead of TCP you using UDP then same above syncronization will happen at "read" system-call and remaining threads has to go back at select/poll. Pseudo code will look like

while(1) {
select(sockfd);
rs = accept(sockfd);
if(rs == -1) continue;
thread_function();
}

But from past-experience. I'd recommend you to re-design of your software with producer and worker concept. One thread will read data on socket then it will select a thread from pool and assign work to that.
From our statistics I'd say 1GHz CPU can efficiently hangle 25 complex threads only!!! And for multithreads instead of deploying higher capacity of CPU you should go for higher multi-core procerrors only!!!