There's routes like the Camino Santiago, where it's apparently pretty easy to meet people while walking. Though most people don't camp, they sleep in hostels.

Also a friend of mine put up a post on a bunch of outdoor/backpacking oriented Facebook groups, looking for people to hike with. She had a HUGE response, like 60 people, picked a few people to go on introductory hikes with, and I believe she found a couple new friends to do hikes with.