We have easily scaled to tens of thousands of 'listeners' on a single node.js process. I am not sure how many you actually can scale to, given that each socket is a file descriptor, and the plain vanilla kernel can have 65K fd's for each process, no more.
CPU would not be a problem. If at all, upload bandwidth would be (1KB * 50K users / 3 sec = 50M/3sec = 16MB/s upstream. I never measured Heroku, so don't know if they sustain this. I suppose they do, but maybe they limit you, since they are paying Amazon for this, after all).