Dec. 21 (Bloomberg) -- Facebook Inc., the world’s biggest social networking site, will overhaul its service in Europe over the next six months as a result of an investigation into how the social network handles personal data.Facebook “has agreed to a wide range of best practice improvements” to its service that will get a formal review in July, the Irish data-protection agency said today, after concluding a three-month audit. Facebook’s Ireland operation is responsible for all the Palo Alto, California-based company’s users outside the U.S. and Canada, the agency said.