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@dalias It's basically the nature of the mobile ad ecosystem in general. When you visit a website with your mobile, in a microsecond the ability to place a certain ad in front of you is put out as an automated bid request to hundreds of ad networks that can all bid on the ability to show their ad. There is a robust market now of participants to this real-time bidding market that simply collect and resell all the live bidstream data, which can include the phone's unique ID, precise location coordinates, and enriched data from other marketing and advertising firms that provide more details about the user.

If you're really interested in learning more about how it all works, you could do a lot worse than to read my linked story, which explains it in more detail.