With Facebook’s S-1 filed with the Securities and Exchange Commission for what is set to be a blockbuster IPO worth at least $5 billion in May, Mark Zuckerberg is now firmly sitting among Silicon Valley’s top chief executives, if he wasn’t already. And, probably with that in mind, Zuckerberg is falling in line with a tradition among some of the valley’s top CEOs: a $1 annual salary. Zuckerberg received a $500,000 salary…