Yes, and no. When people withdraw money, I think you mean they are selling their Steem, and this does produce downward pressure on the price. We don't create Steem when we post. New Steem is created through inflation, not any kind of mining, anymore.

As long as Steemit creates more users of Steem than are able to be satisfied by the amount of inflation, the price is pushed upwards, in theory.

Also, as people power up, or just buy Steem for HODL, the price is pushed up.

The competing pressures create the price of Steem, and it's been pretty stable around a dollar lately. That's the basics of it, and why Steemit 2.0 is likely to see the price skyrocket.