Theories on why IMDb message boards were closed (and wiped)

I've never understood how the boards don't generate revenue. In my observations, the board users were the people who stayed the longest on IMDb. Back when the boards were around, the horror board, for instance, would have contests where members would find, rate, and review movies on IMDb, in doing so spending a lot of time using the site. Isn't that a good thing? That's a lot of exposure to ads. I can see how this kind of interaction might not be measurable, but it did happen. I would think that the amount of time users now spend on the site is far less sizeable.