The '777' on him comes from numerology, so this basically means nothing.
In the same way, his '1998' signature comes from numerology, so again this basically means nothing.
Michael Jackson used these number symbols because they easily stick to people's mind, and also because he had a fascination with numerology.
It has also been said that the singer poses as the number 7 on the front cover of his 'Blood On The Dance Floor' album, and he also forms (with his arms and legs) the number 7 at one point during the 'They Don't Care About Us' live performance (when he has his mouth widely open), which are 2 another examples of his fascination with numerology.