As of Sept 20 2015, the Federal Reserve is still buying MBS at the secondary market on (almost) a daily basis.  The most recent operations are listed on the New York Fed’s Permanent OMO page.

This is part of Agency MBS Reinvestment Purchases and Treasury Rollovers program, where principal payment of agency debt and agency MBS held on FED’s balance sheet will be reinvested in the form of mortgage backed securities, and maturing Treasury bonds will be rolled over at auctions.