Who is GLaDOS's "only friend"?

At the end of Portal 2, GLaDOS sings "goodbye my only friend", but then follows up with "oh, did you think I meant you?" Did she really mean Chell, or was is Caroline? The Companion Cube? Wheatley? Sadness that Cave Johnson is gone? (which might come from the Caroline part of her personality) The bird? The potato?

GLaDOS is refering to Caroline. Here's why I think so:

1. In the previous verse of "want you gone", GLaDOS mentions that Caroline is "in here too". This implies that Caroline is a part of GLaDOS, but not GLaDOS herself.
2. Right before the ending song(s), GLaDOS says that she now knows where Caroline is in her brain. This is immediately followed by Aperture Science's computer announcing "Caroline deleted", and a marked change in GLaDOS's attitude. This was GLaDOS deleting the part of her that was Caroline.
3. A few lines after singing "Goodbye my only friend", GLaDOS sings "Maybe when I delete you I'll stop feeling so bad". This implies that she's addressing the person she deleted, namely, Caroline.
4. Right before the final battle, GLaDOS hears "her own voice" acting as a conscience, preventing her from betraying you. It makes sense that deleting the voice of your conscience would help you stop feeling bad. This harmonizes with the point above, suggesting that it was the more human "Caroline" side of GLaDOS that she was saying goodbye to, and then deleting (so her conscience would stop bothering her).

Caroline was a part of GLaDOS. Perhaps Caroline's personality was merged into the master system in a way similar to how they modified GLaDOS' personality by adding core modules to her (although not exactly the same way). Or perhaps GLaDOS started off as Caroline, but mutated or was modified into something more. Either way, the more menacing side of GLaDOS decides she no longer wants her more human "Caroline" side, and deletes her. She says goodbye to her in song as she does it.

I don't completely believe this, but here is a possibility I have come up with while reading these comments:

GLaDOS could be trying to cover a slip up. When she says "goodbye my only friend" she might actually be talking about Chell, and then adding the "do you think I meant you?" to save her image. Some evidence of GLaDOS considering Chell a friend is right before she deletes Caroline when she says "I thought you were my greatest enemy when all along you were actually my best friend".

Once again, this is only a possibility and I don't completely believe it myself but I would love to hear what everyone else thinks about this.