I didn't make this one up, I swear (but it sounds like something I would)
Leave a list of fictional characters in your journal that you would love to get a message from. It is your friend-list's mission, should they choose to accept it, to write you an in-character "letter" from a character on that list. Then they post their own list in their journal and the process continues!
Harry Potter Severus Snape Hermione Granger
NCIS Gibbs Abby Tony
Star Trek: The Next Generation Picard Data Worf
Pride and Prejudice Mr. Darcy Lizzie
I could probably come up with lots more, but I'll leave it at this.