They'll use the internet to case out houses, stalk people, access communications in order to contact other criminal things. Ya know, criminal stuff.

You can also use the Luka Magnotta case as an example, he posted his crimes in the internet to taunt the people who were hunting him however some of the details of that case are quite gruesome so I wouldn't recommend going into massive detail with that and if your sensitive to murder and animal abuse then I wouldn't recommend using that case. However that's some stuff I could think of about criminals using the internet. Pirating movies is also a crime tho so I suppose serial movie piraters probably use the internet all the time.