Last year, a family of three earning $111,000 a year could afford just 23 percent of homes for sale in San Francisco - mostly in southern neighborhoods, including Bayview-Hunters Point.The median price of a house in the city in 2010 was $668,000. Just 2 percent of new housing units built in the city since 2001 are single-family, detached homes.These were just a few of the scores of statistics presented at a special Board of Supervisors hearing Thursday to help explain why San Francisco is bleeding families with children - losing 5,278 people younger than 18 between 2000 and 2010, according...