Nankay, Tom Riddle was Lord Voldemort as a boy. This is what I think: Tom Riddle's memory, as contained in that diary, was "killed" or destroyed at the end of Chamber of Secrets.
I admit to being fuzzy about Tom Riddle's status. The whole time I was reading about Riddle I had to remind myself that this is Voldemort before he transformed himself into Voldemort. He just seemed like another bad guy, separate from Voldemort.
So did Voldemort-in-hiding become aware that his magical diary had been distroyed? Is this like one of those time machine things, where you go back in time, change something and suddenly the event appears in your memory where it wasn't before?
At the end of Order of Phoenix when Voldemort and Dumbledor are battling it out, Dumbledor calls him "Tom" at least once. Which must make Voldemort furious, because with that Dumbledor is refusing to acknowledge Tom Riddle's power, and puts him in his place as an inferior.