Well, for one thing it really depends on the game. Shooters generally have far more people using mics then other gametypes. Combined with the fact that Sony didn't bundle a mic with the PS3 and there you go.
However, Xbox Live has the same issues sometimes. I would also take notice of the time of day or even what day it is.
People aren't normally very chatty at 1:00 a.m. except on Saturday nights, when you'll get a few drunks on. Another thing that influences mic use is the sales of the game. Expect to find more people with mics on a game that has sold 5 million than one that's lucky to sell only 1 million.
That's about all I can think of. It's not a bad trade off, as many use there mics for very "muteable" talk. So the less I hear of idiocy, the better enjoyment of my game.
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