How about Twitter spying on people? I've set up a Twitter account with my Hotmail account years ago, but I do not use it. Twitter have kept sending me emails about feeds they thought I might be interested in, which were usually related topically to a handful of feeds I'd subscribed to, all of which are based on my academic interests (and are not from my local area).
However, today there was a new twist: Twitter sent me an email saying I might be interested in the feeds of my local council. How on earth did they figure out where I live? I barely log on to Twitter (maybe 3-4 times a year to do a search), and I only do it on my PC. Is that based on my IP address or they're doing something different?