Facebook Inc. doesn’t yet have an intelligent assistant, like the iPhone’s Siri. But the social-networking company says it’s aiming higher, in what has become one of the biggest battles raging between Silicon Valley’s behemoths: How to commercialize artificial intelligence.The once-niche field is aimed at figuring out how computers can make decisions on a level approaching that of human intelligence. Apple Inc.’s Siri, Microsoft Corp.MSFT -1.33%’s Cortana and Google Inc.GOOGL -1.77%’s Google Now are all early manifestations. They are voice-recognition services that act as personal assistants on devices, helping users search for information–like finding directions or rating nearby restaurants. Both “learn” from their users, adapting to accents, for instance, and learning from previous searches about users’ preferences.