Make an ongoing promotion advertising existence of Strava Flyby. It seems that many people are simply unaware of its existence. And it looks like Strava already has a mechanism in place to advertise features by presenting info about them in the feeds. That would give a chance to bring back Strava Flyby from the dead after it was killed a few years ago by forcibly disabling it for everyone.
Strava Flyby is (was?) one of the best features of Strava, which allows people passing by each other to connect or simply say "hi!".