This is a newer site, with an ambitious name: Billion Graves, at www.billiongraves.com. It looks like only North America at this time. Registration with email and password is required. There is free access for basic features and a subscription level for advanced searches.
The service is available as an app for iPhone, iPad and Android. You are expected to use your phone or tablet to take headstone shots while the device captures your GPS location. The phones can upload the photos automatically to the website, depending on the app settings. Once back home, you log into the website to verify your photos and add information on the photo, such as headstone text, names, birth and death dates, and anything else. The app knows which graveyard was visited using the GPS data. Google satellite maps are used to show the headstone locations.
I tried it out and found 3 COMPSTONs: 
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