Facebook blues occurs after writing on someones wall, note, or status, and they don't respond. You wait and wait for a response constantly checking your Facebook to see if they have done so.
After the first day of hourly checking your facebook, you begin to facestalk
them. This includes checking to see if they have commented on someone elses wall, status, or photos. As a last act of desperation, you send them a private message repeating the original post.
Your sadness resonates throughout Facebook until they respond, at which point you've gotten rid of the Facebook Blues.
She wrote on her crush's wall and waited a couple of days for a response. After the third day she was hit by a bad case of Facebook Blues; and, she felt sad that he had updated his status, but not responded to her comment.
Used to describe the shade of blue used in Facebook's layout.
Girl: What shade of blue should I use for the sky?
Guy: Facebook blue!