I am going to go *way* anecdotal on this issue. As a graduate
student at USC, I was invited to present my master's thesis to
a very small conference on ground tracking software at Aerospace Corp.
I got to meet Col. Gillis(?) who was responsible for every single satellite
the US put up into space with a $40 billion a year budget in addition to
tracking decaying ones. He said that the US (circa '93) tracks approximately
170,000 satellites, 24 hours a day, 7 days a week.
Of course, Satellite is defined (his words) as anything larger
than a lug nut flying around up there. They use this information
to route shuttle flights, time safe space walks, and other things.
He also said that each shuttle flight has an average of 7-10 collisions.
Just a story, no URLs. There's a lot of junk up there.