SAN FRANCISCO — Google and Facebook are among the leading marketing channels in real estate, perhaps rivaled only by listing portals in popularity. Here are three pointers for agents that Google and Facebook representatives and a real estate marketing expert shared on stage last week at Inman Connect San Francisco. 1. Use Facebook’s ‘Custom Audience’  Facebook’s Custom Audience feature lets advertisers — including real estate agents — upload contact lists to Facebook and then push ads to those contacts on Facebook. It’s what “we really think of our secret sauce,” said Keith Watts, the industry lead for financial services and real estate at Facebook. Facebook can also build “Lookalike Audiences” based on Custom Audiences, so advertisers can target potential…