How do sites like respond.ly work - support for many longpoll or comet connections?

Question

API

With respond.ly, you can "listen" to what you think about yourself on twitter, as well as receive and reply to incoming messages. Moreover, both in real mode and not: if a message has arrived, a notification about it will be delivered by mail.
For non-live mode, I imagine it in such a way that some services should be running on their side that would periodically poll twitter and send alerts. And how does the mode of receiving messages in real time work? How does it scale?

Answer 1

Subscribing to socket channel messages is a normal socket server. (for example, node.js...)
About scalability - read the documentation for the selected socket server.