What is caesium-137 used for in fracking?
Caesium-137 is not used in the fracking process.

Caesium-137 is used as a source of gamma-rays in some logging tools, most notably the density tool (one example), and some other instruments such as flowmeters. This is what the story you linked to is about. Logging tools are used after drilling to measure the properties of the rocks in the borehole, and thereby calibrate geological models of the subsurface. These sources are valuable, and should be secured and only handled by trained engineers. The caesium is completely enclosed in the tool, and none is left in the subsurface, unless something goes badly wrong with the logging process and a tool is lost.

Caesium-133, the naturally-occurring non-radioactive isotope of caesium, is sometimes used in drilling and fracking fluids in the form of caesium formate brine. Its high density (ca. 2300 kg/m³) improves drilling efficiency and rig safety, and assists with pressure build-up for induced fracturing. It's still pretty valuable stuff, so companies often rent it, retrieving as much as possible (5–15% losses are common, however) and paying only for what they use. Cabot is one vendor of such brines.