Anne Bakstad and Ed Cohen are starting to feel as if their family of four is an endangered species in San Francisco. Since the couple bought a house five years ago, more than a dozen families in their social circle have left the city for cheaper housing, better schools or both. The goodbyes are so frequent that Carina, age 4 1/2, wants to know when she is going to move, too. Eric, 2 1/2, misses Gus, his playmate from across the street. "When we get to know people through our kids, we think to ourselves, `Are they renters or owners?...