The Marin Community Foundation, the dominant force in funding Marin County’s social sector, turns 25 this year. Its development—from its early days when it took over the Buck Trust from the San Francisco Foundation, to the supportive role it played for nearly all the county’s nonprofits, to its recent decision to withdraw much of that support in order to hone in on specific initiatives—offers important lessons about the limits and promise of private philanthropy. Can…