My understanding is that drones are preferred for long term/constant monitoring. Spy satellites are not placed in geostationary orbit, so they only have a few minutes of exposure per day to a place of interest as they fly over.

The Global Hawk [1] drone is optimized for high altitude loitering. Operating multiple drones in the area is one way to overcome the problem of running out of fuel.

[1] http://en.wikipedia.org/wiki/Northrop_Grumman_RQ-4_Global_Ha...