Facebook (NASDAQ: FB) has agreed to acquire an Israeli start-up app maker, Onavo for an undisclosed sum. Onavo, which was founded three year ago, develops mobile utility app, helping users to cut phone costs through more effective use of data. According to Calcalist, a financial website, the online social networking giant is paying between $150 million to $200 million. After the deal is closed, Onavo app will continue to remain as a standalone brand. The deal marks Facebook’s biggest ever acquisition in Israel. Shares, however, fell on Monday. At last check, shares were down 2.20% to 48.03.