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 journaland the process continues!
In no particular order:
Radek Zelenka
Daniel Jackson
Connor MacLeod (the Highlander)
John Locke (Lost)
Lt. Kara "Starbuck" Thrace
Gollum
The Lone Gunmen
Carson Beckett
Kenny McCormack (South Park)
Michael (Century Hotel)
Joe Dawson (Highlander)
Prof. Maximillian Arturo (Sliders)
I'm sure I'd think of more later, but I need to stop here. :)