By Dan Levine SAN FRANCISCO (Reuters) – Facebook Inc has emerged as one of the biggest Silicon Valley companies to have resisted rivals entreaties to stop poaching each others employees, according to emails between the No. 1 social network and Google Inc released in court filings. Sheryl Sandberg had just been installed as Facebooks chief operating officer when one of her former colleagues from Google emailed her, according to the filings unsealed late last week. Facebooks aggressive recruitment of Google employees had heightened tensions between the two companies to Defcon 2, top Google executive Jonathan Rosenberg told Sandberg in August 2008.