In the 20th century empirical sociology possessed innovative methodological resources that granted it fairly exclusive access to understanding human social life. However, with the advent of digital technologies and increasing migration into the online world, this privilege started to shift into the hands of commercial entities. People of the 21st century now generate data with every step they take (both physical and virtual), and most of the current internet business models are built on the collection, analysis, and commercial utilisation of such data. The 'Digital Trace Data' left behind by billions of online users present an unprecedented opportunity for the study of their behaviour, characteristics, and social interactions. This article seeks to introduce readers to the world of Digital Trace Data and the three main areas in which such data are used: research, commerce, and surveillance. Examples of all three are given to illustrate the potential strengths, weaknesses, and associated risks. The article also seeks to provide warning of a future in which the largest repository of sociological data in history ends up locked behind the doors of commercial enterprises and government institutions.