How do Facebook games "keep running" ?

I was wondering.. How do many of those "... -Ville" and similar games seem to run while you aren't on the page?

You plant a flower, you come back and it has turned into a tree!

Do they cache the time between your visits and do a sort of fast forward? Or is the game running on a cloud?

Maybe they store a birth variable. A grow equation could be used with (today-birth)

I'm guessing a cron job is being used?

It would be a waste of resources to have the game running whilst you're not there so I guess it would do the fast-forward trick that you mentioned.

Terraria used the same trick when they introduced the plague biome.

Because it constantly spreads, but it would use too many resources to keep the whole level in memory and constantly update it, the onscreen and surrounding areas are kept up to date and when you scroll around, as other parts are loaded, they are checked against a timer from when they were last saved, and updated accordingly ;)