But since this system only incorporates data from the past few weeks the shift towards quite days will be detected pretty fast and thus these days will start to be visible as busy. It will balance itself out I think
Since only a small percentage of the UK population use Foursquare I'd be more inclined to be concerned it might head in the opposite direction (i.e. relatively small numbers of people using this site to identify quiet days to visit are substantially more likely to check in - resulting in more noise than signal)
