"falling on a big city and DETONATING"... nuclear reactor...
no. there was a slight concern that Skylab would fall on a populated area. There was never any concern about it "detonating". And, btw, Skylab did not have a reactor on-board, as far as I could research. Your thinking of the Russian Cosmos 954 which did have a reactor, it burned up and scattered over Canada. One piece was found that emitted 200 roentgens of radiation per hour -- the level which is enough kill a human after a two-hour exposure. That took months to clean up. NASA was charged $400 by a town in Australia for "littering" with some pieces of Skylab, however the majority of the space station either burned up or fell into the Indian Ocean.
http://www.space.com/news/spacehistory/dangerous_reentries_000602.htmlSatellites are not "designed to fall"... it all depends on the orbit they are placed it. Low earth orbit means the satellite experiences drag from the upper atmosphere, drag that must be compensated for periodically. Other satellites that are in geosynchronous orbits may stay there indefinitely. And they don't typically "burn up over time"... if the orbit decays, the satellite will re-enter the lower atmosphere and burn up through the heat of re-entry in a matter of minutes, not hours or days or whatever.
This isn't star wars. This is blowing up a satellite. Satellites are always in known locations and traveling in known directions. You can predict within a few millimeters where a satellite will be at any given time. Launching a missile to blow it up is not at all like trying to intercept an ICBM in flight (as it arcs from launch site to target). And I doubt this is a "screw up" of any kind. Satellites do fail. They exist in extremely harsh conditions and there isn't any regularly scheduled maintenance (at least, for the vast majority).
Satellites are very very poor at spying on the population in general. They are very good at looking at specific known locations (like, say, army bases, navy shipyards, submarine pens, and, occasionally, terrorist training camps). But looking at you in your backyard... not unless someone looking thinks its worthwhile. And there is a long list of other places to look at. Be more worried about the traffic cams and store security cams as far as big brother stuff. Much more worried.