(Reuters) - About a year ago, when it became clear that taking Facebook Inc public was a matter of when not if, Chief Operating Officer Sheryl Sandberg went out and poached Carolyn Everson, then global advertising head at Microsoft Corp. Landing an executive with Everson's pedigree was a coup - prior to Microsoft, she was a top advertising executive at Viacom Inc's MTV Networks and at Walt Disney Co. The hire also sent a clear message to Madison Avenue from the world's largest online social network: We want to work with you. ...