From their website:
Plane Finder works by picking up ADS-B plane feeds used by commercial and private planes to transmit their name, position, callsign, status and lots more.
Our servers add additional information such as departure airport, destination and photos for presentation on planefinder.net and in our Plane Finder apps.The ADS-B data comes to us either from our own receivers or from people with receivers who share their data with us over the internet. If you are a virtual radar hobbyist we’ll be delighted to receive your data for use within the app. For more details on how you can help please see our coverage page.