Facebook, Google, Apple, Rackspace, Salesforce.com, Citrix, Qualcomm, SAP America, NetApp, Intel, Groupon, Intuit, Accenture and Nvidia are among the best companies to work for. Glassdoor.com has released its 2012 Best Places to Work list, the top 50 companies as ranked by their own employees. Leading the list are Bain & Co., McKinsey & Co., Facebook, MITRE, and Google. This ranking differs from that of the Great Place to Work Institute. In naming Microsoft the best multinational employer back in October, it surveyed more than 2.5 million employees and used “workplace culture analytics” to make the determination.  It named SAS the best U.S. employer earlier this year. Glassdoor's rankings, however, require that a company's employees go to the site on their own and submit evaluations. That was pretty evident last year when Facebook was ranked No. 1, though it hadn't been on the list at all the year before. It gained the top ranking with just 34 reviews. A list of CEO approval ratings also was released. The top-ranked CEOs were National Instruments' James Truchard and NetApp’s Tom Georgens, both at 100 percent approval, followed by Apple’s Tim Cook (96 percent), Synopsys’ Aart de Geus (96 percent), and  Intel’s Paul Otellini (93 percent). Meanwhile, approval for Xerox CEO Ursula Burns fell to 28 percent, new Yahoo CEO Timothy Moore has just 31 percent approval, and Microsoft's Steve Ballmer has just 35 percent.