Is it rude online or just in real life?

If someone is polite and says hello to you in real life it would be rude not to respond. Is that not the case online? We have all had that person that makes us a little uncomfortable the tries to start a conversation with us in public. We wave smile and then keep it moving. Is that not the case in the cyber world is it OK to just ignore people? Perhaps I am just older and I am not cool enough to understand the ways of the cyber world but please comment I am curious.