I would say that discussing or creating any non-4chan community is either off-topic or advertising with a few exceptions.

The exceptions to this would be an "organic" kind of high-effort community. Something like this:
>http://4chanmusic.wikia.com/wiki/Sticky
comprised of people compiling information and advice from the users of 4chan itself, for no personal gain. This kind of community is something to encourage.
You also get collaborative projects, for example /lit/ writing and self-publishing a book with the work taking place outside the site, or /v/'s musicals, or boorus for art from drawthreads, etc.

The distinctive thing about these is that they're productive, project-based communities that identify with the 4chan community. a radio stream would probably be okay, as long as it's composed with the 4chan community in mind and is on-topic to the board. Any new community being discussed should show signs of being project-based and productive to the topic of the board, and NOT just "another place to talk about X topic."
Mods should probably be involved if you're having trouble with any of these project communities, like the /k/ patch salesmen or drama about multiplayer servers or something, (i don't know a lot about these particular topics, just using them as examples)

Also, if a site/community is brought up as a passing recommendation, that's not against the rules as long as its not blatant, you can use your own judgement and knowledge on that.

This isn't /policy/ but it's how I do things and how I see others do them, I hope that helps.