"Facebook was not originally created to be a company. It was built to accomplish a social mission — to make the world more open and connected." Mark Zuckerberg giving the keynote at SXSW conerence in 2009. Credit: Jim Merithew/Wired.comJim Merithew On Wednesday, Facebook filed the prospectus for a $5 billion initial public offering. Here is CEO Mark Zuckerberg's letter to potential investors. Face