I've seen women posting on solo travelling facebook groups their current or next location and PM each other, especially in non-english language groups.

With the app, you would receive a notification when a new traveller (speaking your native tongue or not) is close using geolocation, then you check their facebook profile and message them (via messenger).

No need to display the app: you just wait for the notifications (which frequency can be changed). This is my idea for solving the egg & chicken problem, so obviously the app doesn't display ads (and is free).