In 2012 ASUS (22.2 million) and Gigabyte (19 million) combined shipped ~46% of all motherboards. In 3rd place ASRock shipped 7 million, MSI 6 million, and ECS, Biostar, Foxconn, Intel and a couple of other companies filled up the remainder.
As i mentioned in another thread awhile back, Intel is leaving the motherboard business, even though they shipped 5 million units in 2012, putting them in 5th place.
But so far this year, Q1 2013, ASUS has shipped 5 million and Gigabyte has shipped 4.9 million. So close! What will Gigabyte do in the months to come to sway things their way? It'll be an interesting battle for sure.