Good question. Another ARTICLE says Messenger traveled 96 million miles. Much more plausible, eh. Amongst other things it made three passes around Mercury before being inserted into orbit.
And that article would be wrong. The spacecraft is now about 93-96 million miles away from earth, but the actual distance traveled is the 4 billion miles. It took more than 6 years to get to Mercury, as it had to do a whole bunch of orbits around the sun along the way. My previous post has a link to the trajectory diagram.