Matthew Ingram sketches out why Yahoo should be more afraid of Facebook than of Google. (Yahoo CEO Carol Bartz yesterday said Facebook was the bigger threat at a Bloomberg Businessweek event.) Both Yahoo and Facebook’s revenues depend on brand advertisers, and on audience reach, frequency, and duration (time spent with the site, or page views), while Google doesn’t really care if its users hang around. But Google’s the one with an ad network; so far Facebook hasn’t gone that route. Earlier, I wrote how Yahoo could best inject a jolt of social into its system. Google’s eager to do the same, but if this is its master plan, it will need more work.