To mack someone is to
post comments about them on their friends my
space pages. This could involve starting rumors or getting
something of your chest.
Thus leaving the person in question, completly embarrased about what you have informed he/she about, in relation to themselves.
E.
g. I just macked your ass and told
fred you like him! Or, You just got a
good macking of him/her, now everyone knows.