Outside of the State of the Union address, I'm not aware of any "main method of communication" for the POTUS. You could argue the press conferences held by the press secretary are one form, but they're not mandated by the Constitution.
And the State of the Union address was delivered in written form to Congress for more than the first century after the Constitution was ratified. Hoover was the last to never give a speech for the State of the Union.
I would say the press conferences are the main method as they are from the White House and happen multiple times a day. I don't know that a Constitutional mandate would have anything to do with it.
