Privacy is important, but even when you're careful to not leak your info publicly, researchers can still figure out where you are fairly easily. Ars Technica explains how.
Researchers from IBM created an algorithm that predicts a Twitter user's location. It's only 58 per cent accurate, but that's still significant. Here's how it works:
While geotags are the most definitive location information a tweet can have, tweets can also have plenty more salient information: hashtags, FourSquare check-ins, or text references to certain cities or states, to name a few. The authors of the paper created their algorithm by analysing the content of tweets that did have geotags and then searching for similarities in content in tweets without geotags to assess where they might have originated from. Of a body of 1.5 million tweets, 90 per cent were used to train the algorithm, and 10 per cent were used to test it...
The most salient use of location data from a business standpoint would be for targeting ads... it's not implausible that their Twitter actions, even passive ones following other users, could be examined to determine their location.
Obviously, it's not possible to pinpoint where you are exactly, but it's easy to see how someone can figure out your location based on what you tweet about, who you follow, and what you hashtag. So, if you really want to remain private, be careful about how you use Twitter.