Facebook - one word or two?

On a lighter note... I was sending a message via "facebook" and was expressing my frustration that another of our friends did not have their own profile on facebook yet. While typing the message, I noticed that Facebook underlines incorrect spelling with the infamous red line - and in fact had underlined the word "facebook" with a red line.

I thought it was ironic that the Facebook dictionary does not recognise its own 'name' so to speak.