Mon Jun 4, 2012 10:03am EDT (Reuters) - Salesforce.com Inc will buy Buddy Media, which helps big brands manage Facebook and Twitter pages, for about $689 million, in the latest sign that the social media marketing sector may be a new battlefield for large tech companies. Salesforce's biggest ever deal follows rival Oracle Corp's acquisition of social media marketing company Vitrue. Industry website TechCrunch said the Virtue deal was for $300 million. Buddy Media, which allows customers to publish content and measure the effectiveness of social media marketing programs, serves Ford Motor Co, Hewlett-Packard Co, L'Oreal SA and Mattel Inc. The five-year-old company, co-founded by its CEO Michael Lazerow,…