I'm not aware of any (and am interested if there are). In recent years I'm finding that the "current year" per-trail facebook page or group is the go-to place to connect with other hikers while actually on the trail, to pass on important info --- water sources, trail closures/re-routes, trail angel stuff, whatever. It can also be helpful post-hike to stay in touch with folks you met, or even connect with mutual acquaintances who hiked the same year.