How should I say goodbye to my dear friend and coworker? Please help me?

If anyone who has read my past questions you know that I am a guy who is lucky enough to be close friends with this amazing woman at work. We share our secrets, hang out, and share so many laughs. Anyway I found out that I am being transferred to another office soon. Its very complicated because I am not allowed to tell anyone until my last day or perhaps on my first day at the new office. I would tell her on my last day but she leaves on a short trip two days before my last day and she won't come back until my first day at my new office. So when she comes back I won't be there. The new office I am transferring to isn't very far but we work long hours, I go to school at night and the weekends, and she's busy with her kids so we don't hang out after work like we want to.

I'm thinking of calling her on my first day of my new office before she gets to the office instead of texting her. It's hard to choose because she hates talking on the phone with people and prefers texting. She doesn't even like to call her mom or dad and prefers to text. I am also thinking of bothering to find my password to Facebook and write her a heartfelt message. She really likes Facebook and always tries to convince me to log into Facebook so I figured its a good idea. I can't tell her when she leaves for her trip because again, I can't tell anyone. It's complicated but I did not do anything wrong. Please help! Are any of these ideas good? Or please share any better ideas! Thank you so much!