Rivals and skeptics are wondering how Avis will market its acquisition of ride-sharing company Zipcar, which was met with skepticism on Facebook among the brand's fans. The brand has grown through a strong social media following and user evangelization, and Avis CEO Ronald Nelson said that the company is "committed to retaining the elements of the Zipcar brand and culture." But Avis hasn't exactly been at the forefront of social media brand-building, and "this might mean a learning curve ahead," writes Joan Voight.