“Community-Powered Motivation.”
"Community-powered" is abstract. Strava's real value is the network — the chance your friends will see your run and react to it. The hero doesn't make this concrete.
“"The fitness app where your friends are. 125M athletes. Every run, ride, and walk — shared, ranked, celebrated."”
Naming the network size (125M) makes the social effect tangible. "Your friends are" is the network-effect closer. Three verbs (shared/ranked/celebrated) communicate the actual UX of using Strava.