Investors may fret that the car-sharing business of Zipcar Inc. could get gobbled up by giant potential rivals such as Hertz Global Holdings, but Cambridge-based Zipcar can says this: It has more than 80,000 Facebook fans. Zipcar recently tried to mobilize its Facebook fan-base with a social media contest dubbed “The Ultimate Ziptrip.’’ The company asked these folks to explain in 750 characters or fewer what they would do on an all-expenses paid trip to one of the 16 North American cities where Zipcar operates. Lowell’s own Jack Kerouac, a road-trip poet laureate known for windy prose and a disdain for punctuation, well might have taken a pass. But nearly 5,000 Zipcar fans responded to Zipcar’s call. Zipcar next winnowed the field to 50 semi-finalists, who were then “tasked with creating videos to support their cause,’’ Zipcar said in a press release.