Birkenstock warns in IPO filing that knock-offs on Facebook are a top risk

Miami, Florida, Coral Gables Shops at Merrick Park, Nordstrom Department Store retail show of Birkenstock sandals.  Jeff Greenberg | Universal Images Group | Getty Images Birkenstock, the enduring sandal maker based in 1774, filed its paperwork for an IPO on Tuesday, and warned buyers of the dangers posed by counterfeit manufacturers who use social media…