don't u just hate when ur on Facebook and someone IM's u saying "hey" and ur like "hey" and they say "what's up?" ou "what u doin?" and ur like "watching tv" ou something and they're like "cool" and then ur like "yep" and they just don't IM back?
If your trying to ask people if this kind of thing is rude than no it isn't. The reason people don't reply back right away is because they're either doing something and they don't have the time to write back every minute, so don't make it sound like it's that big of a deal.