"Promoted Stories" myth needs to end. It wasn't the case, there was a severe bug whereby a Regular Expression only matched RSS content. The Regular Expression acted as a gateway into the Popular Algorithm. I worked at Digg and I fixed that bug.
It wasn't noticed before launch because we echoed the v3 popular stories into the beta version of v4.
Digg was never paid for stories hitting the frontpage. And for all the flack it gets for this myth, it should have been.
Think of desktop apps that keep a connection to the server and are awaiting messages from it. I think it might get interesting, maybe not the best application of WebSockets, but interesting. Your Email client wouldn't have to poll every X seconds to see if new mail has arrived.
They were ads that were pieces of content. Digg was in front of this trend with promoted tweets / stories.