Standing in front of Francois Payard Bakery at Columbus Circle, Walter Piper shifts his weight nervously from one foot to the other. The 28-year-old is waiting for Jay, a bioengineering student whom he met online just days prior. To take the pressure off of their first meet-up, they planned to run the 10K loop in Central Park together before work. But this isn’t the beginning of a romance — the two men met through an online “friending” site, gofindfriends.com. Piper, despite having more than 300 friends on Facebook, found himself in a predicament shared by many millennials: He had no...