This article is from the Satellite TV FAQ, by Gary Bourgois firstname.lastname@example.org with numerous contributions by others.
The average lifespan for a communications satellite is about 10 years. While
the electronics inside the satellite can last many many years, the
determining factor is the "station keeping fuel". Satellites only "appear to
be stationary because of their location in the Clarke Belt, in reality they
are whirring about the planet, and their orbits become eccentric if left
alone. So each satellite has small rockets on board to regularly adjust
the orbit of the bird. After 10 years this fuel runs out, and the satellite
can no longer be adjusted with respect to its position. This causes the
satellite to start to appear to "wobble" up and down in the orbital plane,
and eventually become unusable. Before this happens, a replacement bird
is launched, and the old satellite is unceremoniously "kicked" up into a
higher "parking" orbit. While it is a nice thought that some day a
space salvage company could go up there and refuel all those old birds,
it is unlikely, and the rapid changes in technology make the older low
power satellites nothing more than curious antiques.