why sometimes this girl appear or disappear on my chat sidebar of Facebook?

[Tyler]

I chat with her recently on facebook. She knows I like her. Sometimes I feel my opportunity is positive but sometimes it is negative. But one thing I noticed is that she disappeared on my chat sidebar of Facebook a fews days ago. After about 5 days, she appears again. The I sent her message and chated with her a little bit and then she disappear in chatbar for 1 day again. Today, after 1 day she appears there again. Is there any possibility I can infer that she sometimes try to make her invisible to me?
Whether she's online or not seems irrelevant to whether she appears in the chat list or not. Oh maybe I didn't make it clear, when I said she didn't appear in the chat list I just mean she didn't appear in the top column, she can appear in the "more friends" column if she's online.

[Lenny]

The chat has an option to where she can tweak to where she appears offline to you if she so chooses that option

[Mackenzie]

she goes offline........