Orbit duration is largely a matter of altitude. The higher the orbit the longer it will last. Many communication satellites are geostationary and will never come down in any reasonable amount of time.

Communication satellites used to carry data where latency is important must be in low Earth orbit, which means their orbits degrade relatively quickly and thus they need to expend fuel every once in a while to compensate for the tiny amount of drag from the very thin atmosphere up there.

I don't know how long they last on average, but any new satellite is required to have the capability to either safely re-enter the atmosphere (i.e. largely burn up) or boost itself up into a graveyard orbit.

I'm sure 100 years would be trivial for a geostationary satellite. I think there is no chance a LEO satellite would last that long. To maintain a LEO orbit, the satellite needs to be almost completely functional. It needs power, fuel, attitude control, and navigation. Solar panels and computer chips degrade in space and nobody puts 100 years of fuel into a satellite that will likely be obsolete in 20.

Small satellites can be launched by ICBMs actually. The Russians and USA have both used old ICBMs to do this. The Russians have even launched them from ships and submarines. Only nation states have ICBMs though. There are several private space companies currently working on launching satellites.

Cheap, small satellites would be launched either by the Russians aboard an ICBM, or aboard something like a Pegasus. http://en.wikipedia.org/wiki/Pegasus_(rocket)