By Yasmeen Abutaleb SAN FRANCISCO (Reuters) – Facebook Inc is testing a personal digital assistant called “M” within its Messenger service that can answer questions with live human help and perform tasks such as buying gifts online and booking restaurants. M is “powered by artificial intelligence that’s trained and supervised by people,” David Marcus, vice president of Messaging products, wrote on his Facebook page on Wednesday. Rival services like Apple Inc’s Siri, Google Inc’s Google Now and Microsoft Corp’s Cortana rely entirely on technology to answer questions.