Most of the apps are "listening" for specific system events, which cause them to perform some actions, killing such apps (e.g. using task killers) to save battery is counterproductive, cause in the end your device will consume more battery due to constant killing-restarting cycle.
To manage the problem you could use one of the two apps (maybe there's more, don't know), Autorun Manager or Autostarts, both available in market. Using either of those two you can disable the events causing apps to restart, usually without damaging their functionality - for example, if you want to use fb only when you need it, you can disable all triggers, and the app we'll run only when started by you, and will not restart when killed.