To BMW salesman Robert Becker, serving customers hot dogs at a sales promotion event was “absolutely not up to par with the image of the brand,” and he mercilessly mocked his employer, a Lake Bluff, Ill., dealership, on Facebook for doing so. He was promptly fired — though the dealer denies this was the reason why.

Becker’s fate is hardly unique. Scores of other employees who made negative comments about their employers on Facebook and other social-media sites have found themselves out of jobs.