I'm an introvert. I like my friends, I just don't always feel like talking to them. If I hide my status from anyone, it's usually because they are too extroverted. If anyone besides my lover IM's me within the first 5 minutes of me logging in, and it's not due to some urgent matter, then I will probably take away their ability to see when I log in. When I'm in the mood to chat then I will go somewhere public where I know people and I will chat. I very rarely initiate an IM with anyone unless there is something specific I need to discuss with them.
I've had people get pissed off at me before for hiding my status. I just tell them that friendship doesn't mean giving away one's right to privacy. If they don't accept that, they are free to unfriend.