Yep, they are in Low Earth Orbit. A place that has a very, very small amount of air, so the satellites experience drag, lose speed, eventually the propellant tanks run dry, and they burn up in the atmosphere. The ISS experiences the same thing, which is why its altitude slowly falls, then you see a sharp increase as they push to a slightly higher orbit.

At the altitude the SpaceX satellites are at, they only passively stay up for a few years. With the onboard propulsion giving them each another few years.