Well, couple of posts ago - Pokersam posted link to redemptions of mutual funds which are baskets of stocks?
Yes, on an individual level just an exchange...bigger picture, instead of putting that cash back to work, one may have redeemed to pay bills or whatever...
No way a constant. On a macro picture, money is put into system by Fed, in a growing economy or directly by the FED(as is now). Some makes its way into market. In slumping economy, the opposite(unless FED steps in).