Rent surveys are usually heavily biased towards large buildings. SF has a ton of new development in currently moribund neighborhoods, so I suspect some of the massive drop is due to that being oversampled (Mission Bay, for instance). In the more desirable neighborhoods with lots of rent controlled places the price drops likely aren't as steep. My friend is trying to rent a condo in a desirable location and most people looking at it are moving within San Francisco, trying to get into the more established neighborhoods at a better price.