Backpackr is an app that helps solo backpackers meet other people while they travel. I can vouch for solo backpacking being an excellent way to travel and meet people, but traveling alone can also be lonely if you're not good at putting yourself out there and meeting people.
The app allows you to browse through profiles of people traveling to the same places as you and message them. You can filter your results by age, gender and location. This app is a great way to find people to travel with or just hang out with.
Many other travel apps and services often get hijacked by people who use them to look for hook-ups, but the developers of Backpackr are projecting the app as strictly a travel buddy-finding service that has nothing to do with dating or hook-ups.