You compare a pull to a push there. Sure on a service where you pull the information, you wouldn't pull what you don't want. Internet is a good example of that,

You compare a pull to a push there. Sure on a service where you pull the information, you wouldn't pull what you don't want. Internet is a good example of that, your example of twitter-like is also pull. It fail when it's a push instead. That would be anything that have suggestion, any public conversation, like a forum, or twitter, even private conversation in some case, like an email service, etc... For sure you won't pull what you don't want, that's by definition, but you moderation is needed for the push system, where you don't control everything that goes to you. Most service are push too, we want things to be suggested, we don't always know exactly what we want, we don't know even if it exist.