Stratospheric growth in coworking facilities has spawned copycats across the city—and possibly the next big real estate bubble. Manhattan has a record 5.3 million square feet of shared office space, according to real estate services firm Cushman & Wakefield; and Brooklyn has about 1 million square feet, with another 400,000 feet to come in the near future, according to a report from New York University’s Rudin Center for Transportation. There are 53 co-working brands in Brooklyn alone, according to an industry report, and at least 70 in Manhattan.