Scanning the website, it looks like it depends on you having populated the user's Facebook, Instagram & Twitter profiles in order to use those services. If you have an email address, it'll use Gravitar, and if you have none of the above, I guess it does nothing? Given that Facebook already takes a full copy of all of my contacts (I use WhatsApp) I'd really rather not spend the effort curating a more detailed database…
I recommend using web apps, not native apps, for things like Facebook. Your browser will not hand over your contacts list to the world, so preferring your browser is often a good move. Won't work for everyone, but it works for me.
You can also choose apps from orgs that care more about privacy. Signal works just fine instead of Whatsapp.