It should be obvious if they're being rude. Otherwise, its difficult and wrong to jump to conclusions about the other options. There have been instances where people say they've sent me PMs which I never recieved, and people that were wondering when and if I responded to PMs, when I did, and they supposedly didn't make it through to them.
Now, as for people that are ignoring you by not responding, I'm not sure if thats rude or not. First off, if they're harassing, trolling, or baiting you then its a given to not reply after a point or it's revealed what their intent is. Its rude when they stop the conversation and leave without saying so. Though that can have it's own set of circumstances (an emergency, no time to go on and explain why, their connection fails and they can't return, etc.)
But then theres people that feel that no reply is necessary. I mean, when do you end a conversation and who does it? You know? It sort of an unspoken, informal thing. One person either says goodbye or goodnight or whatever and the other doesn't reply again. Thats standard procedure, I believe.
If its a conversation they intend to carry on to the next day or next time, one should mention it and the other confirm it.
Also, people need to understand time zones and their affect on replies.