It means they can't be bothered instant messaging, but they don't hate you. I'm the same.
As I have an attention problem. I'm more prone to be myself when shoved in front of people and want to make a good and friendly impression, and interact well in a social scene.
As far as instant messaging goes, I see it as a chore with most people, and have no inclination to engage in it whatsoever. As it's the internet, it doesn't put you on the "spot" as much as being thrown into a social situation in real life, (for example - you can set it to away and just not bother having to discuss stuff, or just not reply and people don't really care as much, unlike real life you will be seen as rude and unfriendly instead of "busy") so you can just brush it aside.
Also might I add that talking over a computer removes the basics of interaction such as tone, warmth, general vibes of a person, and detectable emotion.
You can zig zag around social expectations with internet communications and you can't in real life.