Fannie Mae and Freddie Mac continued to be major investors in the rapidly growing non-agency mortgage-backed securities marketplace last year, as their product choices became more diversified. According to data compiled by Inside MBS & ABS, an affiliated newsletter, some $274.6 billion of securities issued in the booming non-agency MBS market last year were backed by conforming balance loans and structured for sale to the GSEs. That was almost 23 percent of the $1.2 trillion…