What happens to satellites that have outlived their usefulness and have been abandoned?
They keep orbiting the earth. If they are not very high — about 400 kilometres or less, they lose velocity in course of time due to friction with dust particles, and gradually spiral towards earth, burning up when they enter the atmosphere, though sometimes pieces may hit the earth.
Those that are thousands of kilometres above the earth never lose velocity and may continue in their orbit for thousands of years.
Once satellites have completed their mission and are about to be abandoned, their electronic transmitters are switched off so that their signals do not disrupt transmissions from other satellites.
The International Telecom Union would like satellites that are low down to be pushed up before they are abandoned, so that they do not fall back to earth. But most space nations fail to do this either because the satellite has exhausted its fuel by that time or they've lost control over it.