S'mon said:
As GM I don't keep a journal, but I give players XP for what they write. Seems to work fine.
I think a little honey like that helps. I've been running a homebrew world since 3.0 came out and last year added a wiki. They all know I'm open to players fleshing out the setting around their characters, and people can put into it what they want and I review it and it's official.
If anything, I've got one of those "I wish I had that problem". One player who's tuned into my vision, creative and prolific. I've actually had to talk to him about doing in-game rewards like plot hooks instead of XP because he's done so much and I don't want to further widen his XP lead because of out-of-game stuff.
One the other hand, I have some very creative players who expand the world in-session but don't write on the wiki more then their character pages. And that's fine too. This is a game, forcing players (and unreasonable expectations and pressure ARE forcing) to write moves it from fun to work. Who wants that?
Since I'm in a little different position, where players are adding to the world instead of journaling what went on, I have the extra bonus of trying to work in things that they've written. My players get a big smile when I refer to places or myths or what-have-you that they came up with.
So I agree with S'mon - carrot works better then stick. And I'll add to it that different people may or may not enjoy/have time for/have talent to do out-of-session writing, and rather then taking your players to task, you need to accept it.
Good luck,
=Blue(23)