Some answers in @steemitblog article

Who gets the Money?
The @null account is a special account which no one has control over. This means that once money is transferred to @null it is locked away for ever. In a future hard fork we will remove the Steem Dollars, Steem, and Steem Power held by this account from the supply, but until then it is effectively gone.

This means the money is distributed to all stakeholders. Since all payments are made in Steem Dollars and Steem Dollars are similar to the debt of the network, it means all payments made to promote posts serve to reduce the debt load on Steem which increases the value of Steem.